For one, because of physics: it's easy to hit a slow moving tank from a flying vehicle. It's impossible to hit a fast-moving flying vehicle from a slower moving one. And you can't make a cheap light rotor-based drone move as fast a jet-engine-powered plane no matter how hart you try.

For another, R&D: we've been working on fast autonomous flying vehicles to take down aircraft for decades: as the poster above keeps explaining, they're called AA missiles, and are extremely expensive, and even still have a hard time actually hitting a plain. Drones are an innovation in fighting tanks because they are much more maneuverable than traditional AT weapons and thus can more easily find chinks in the tank's armor. You could have always used the extreme maneuverability of AA rockets to hit those same chinks in the armor, but the cost was too high; drones were cheaper. AA rockets are extremely cost effective against plains though, as those jets are monumentally more expensive than a tank.

The issue is that the F-35 when it's actually being competitive is practically invisible. The US actually has generally had to operate it without stealth coatings or while broadcasting a radio beacon while doing comparisons or competitions against other crafts and weapon systems because in an actual combat environment, an F-35 fit for stealth is completely invisible to electronics (and to a measurable degree eyesight) well within F-35 weapon systems' effective ranges.

So essentially you can't see an F-35 until it is already lined up to kill you. That tends to make it pretty difficult to hit it.