Sure, for an individual it reduces the chances they'll miss their flight. But if everyone does it, there's obviously no change. The only way showing up earlier would help with throughput is if there are periods of time where the security is under-utilized with absolutely no queues. Which, from my experience with US airports, is a laughable idea.
Short lines are nice, but it's also not bad the have direct flights to Europe and many US cities, and pretty low ticket prices.
Due to the frequency of travel, I do have Pre-Check, but I hate that I had to pay extra money to get what I think should be the normal experience for all domestic travelers.
It's nicely described by the Prisoner's dilemma.
If there were no Pre-Check, normal lines would still be long—they'd just be slightly shorter.
I'd much rather pay for dramatically shorter lines than to save that money and have slightly shorter lines in normal security.
EDIT: It would be good both for this conversation and HN in general if the downvotes were explained. The parent states several things that in my opinion are not completely true (such as all people arriving earlier don't having effect on anything, or the need to have absolutely no queues at some times for throughput to be improved, or that that never happens) and pointing some things out doesn't seem to me like a punishable action, but if it is, some explanation would be good.
In particular, my experience with for example the Frankfurt airport is that you get lots of security lines but more than half are closed, with up to 8 employees per line. They require that no one starts unpacking and preparing things until the area is cleared. They request a chemicals test if the metal detector jumps on you (?). They do random things like what vincefutr23 points out in another comment: > still having to take belts and shoes off even with a full body scan
The effect of all this is that you get more "latency" than "throughput" problems, as each particular line clogs a lot, and you could open more lines if you wished, and it takes you a lot of time to be serviced even with few people.
But then again: it only is the problem of the airport if they are affected by it - but they don't seem very keen on improving it.
Now you change that, asking people to show up 24 hours before their flights. At 9:00 on Monday, the airport holds all people with flights departing between 9:01 Monday and 9:00 Tuesday -- 24 hours' worth of people, eight times as much.
Airports can't hold 8x as many people without being in violation of the fire code.
Unless everyone is abandoning the line and going home since they're going to miss their flight (unlikely).
Shrug, I've seen this plenty over the course of my life in the US, and I've only ever really flown big-city airports. It's not ubiquitous, but it's not like, super-rare or anything.
Show up earlier, and if they process everyone earlier, everyone will get through in time.
Of course, this does not help you get to your connection on time.