They're working on it. Can't ban it since many airplanes still require it.
Alternatively, introduce a Pigouvian tax: charge an obscene and increasing amount to burn leaded fuel.
This is kind of like the regulations that permit grossly polluting old collectible cars to operate. Sure, they have history, but that’s not a sufficient excuse to allow them to operate unmodified near other people.
Generally speaking, airports are reluctant to spend money on a second set of tanks, pumps, etc. required during a transition period; though some have - particularly in the midwest where there are a lot of less regulated homebuilt aircraft that can burn alternative fuels.
A lot of pilots would love to burn something else. 100LL is relatively nasty and builds up in engines, shortening their life. It's even actively discouraged to burn 100LL in some more modern small aircraft engines like the Rotax 912 and pilots like myself who run that engine look for non-leaded fuel whenever practical because it's healthier for the engine. But well maintained aircraft last just about forever and the legacy fleet is absolutely enormous.
It sounds odd to put it this way, because leaded gasoline started being phased out over 40 years ago. In an alternate universe, we would now have 40 years of safety data for unleaded and for leaded.
When lead was banned in auto gas, it was replaced by MTBE, which itself was banned in most states by 2007. The alternative to MTBE is Ethanol, which is currently used in auto gas. But Ethanol is incompatible with aircraft because it is corrosive to aluminum (and tends to cause more serious vapor-lock problems in fuel lines).
In other words, there's no widely-used blend of fuel that could even potentially have 40 years of safety data. Pilots are already used to paying a lot for fuel (more than auto gas), so there's money at stake if someone can come up with an unleaded alternative. It's just that someone has to invent that safe alternative first.
On top of that, fuel is a particularly sensitive issue to the FAA because fuel and engine malfunctions are currently the 3rd and 4th leading causes of aircraft accidents (and this is after the FAA has spent decades on safety programs to reduce fuel-related accidents).
Incidentally, this is also an issue with small engines in things like garden equipment --- the ethanol tends to degrade plastics over time too.
Also, I happen to have a car from the 80s and in looking for information about the consequences of using fuel with ethanol, some say it can be a problem while other people say at that point in time it was designed to handle it. So I'm not clear on what diversity there was in the types of fuel available over time.
That's moving the goalposts.
Piston airplane engines typically (but not universally) have high compression ratios, so they require fuel with a high octane rating. In fact, the only avgas that contains lead today is 100LL (100 octane, low lead).
If you want to have a discussion about lead in avgas, you're having a discussion specifically about 100 octane fuel.
> What about iso-octane?
Definitionally, 100 octane fuel has anti-knock properties similar to 100% iso-octane. You may be able to make an approximately-100% iso-octane blend for lab tests, but it's not really possible to manufacture it in commercial quantities.
Keep in mind, "premium" auto gas is usually less than 93 octane, and even that has ethanol.
Everyone in aviation wants to get off 100LL fuel, we're just waiting for the FAA to certify one of the replacements as safe.
But in aviation you're not only worried about the knowns, but also the unknowns. Example: JAL 123, BA 38, TWA 800 and other cases where you believed things to be safe but in reality they were a ticking time bomb and/or would trigger in very weird conditions.
A couple years ago we got close to having an approved alternative - Swift Fuel's UL94. However as an example for why this is hard: one of the objections to that fuel was that it didn't weigh the same as 100LL, meaning its use would alter weight and balance designs of the aircraft that use it. For most airplanes that's not a big deal, for some it is.
We have 80 years of aircraft flying around designed more or less around the characteristics of one specific fuel blend and most of those aircraft have decades of life left in them. We all want to get off 100LL, but we also don't want aircraft falling out of the sky as a result.
The FAA is required to evaluate the impact of new regulations on the existing fleet. A change that would eliminate or place a prohibitively high tax on leaded fuel would likely be shown to eliminate half of the GA fleet. This will not be approved until the impact can be reduced. Developing and certifying new engines and fuel system components for all the different aircraft type certificates is totally infeasible; a new fuel substitute is pretty much the only option.
The FAA has made huge regulation changes before but there has always been an alternative. The recent ADS-B mandate requires about $5k of new equipment before a plane is allowed to fly where Mode C transponders were previously sufficient. This is/was expensive for many private pilots, but was deemed to be acceptable for the safety benefits gained. Some owners have chosen not to add the ADS-B equipment and haven’t been allowed to fly in some parts of the country, but they can still fly most places. A fuel regulation that grounds half the fleet with no alternative regardless of the price is a completely other level of impact.
The FAA and most pilots want an alternative to leaded fuel (at least for the assumed cost savings if not for the environment). Energy companies are working on unleaded substitutes but current options still require an STC to burn. You probably won’t see leaded fuel going away until there is a universally approved replacement that can just take the place of 100LL at every airport in the country. Edit: or barring a complete drop-in replacement, at least an option that can be shown to work in existing engines and fuel systems without requiring major R&D.
Lab specimens, search and rescue, law enforcement, aerial photography for the "satellite" maps on your phone, power line surveying, transportation to remote parts of the country. Not to mention training for both future airline and military pilots.
Just because they're not economical for passenger service, doesn't mean they don't have an important role.
And again, I don't think you'll find many people in aviation who wouldn't like to move away from leaded gasoline. They're just waiting for the FAA to certify a replacement.
Yes it's bad. Yes we need to get away from it. Yes it's being worked on and will happen eventually. Also please at least recognize that it's a hard problem.