The reason is that neither side has air supremacy. Ukrainian AA defense is good enough to keep the Russians at bay, but Russian AA defense is also good enough to prevent Ukrainians from taking out their frontline defenses.
So with classic air forces being all but taken out, the only way either side can make progress is by using tanks and artillery.
[0] https://aviationweek.com/defense-space/missile-defense-weapo...
[1] https://www.eurasiantimes.com/hamrless-missiles-us-supplied-...
It definitely could provide much more to Ukraine if both parties were aligned to the common cause of sustaining America's hegemony by being a reliable ally, right now there's one party which the whole ideology centers on going against whatever the other party does and/or supports. Even if that means allowing Putin's Russia to gain more power and influence.
I don't think the vast majority of Americans understand the long-term consequences of allowing the USA to become unreliable to its closest partners (the West in general). You will be feeling this over the next few decades, America's soft power is waning.
It's even worse. The 45th is actively calling for Russia to take what they want.
Basically the problem is that to avoid AA you have to fly low or far from the front lines. If you fly low, you can't give enough energy to your missile to threaten even just Russian bombers.
If you fly high but far away, there is no way to deal with Russian planes carrying R-37s that will be able to fire their missiles far before you.
The only way to even the playing field would be to give Ukraine modern Gripens with the Meteor missile, as the F-16 cannot fire the Meteor.
Their role will most likely be to fly far behind the frontlines and fire NATO weapons Ukraine's airframes can't.
