Why can't those be taken out, like the "highway of death" in Iraq?
One reason is that you have to have absolute air supremacy; that's when the enemy is incapable of effective interference with your plans. We had that in Iraq, total air supremacy. Air Superiority isn't enough. That only prevents the enemy from prohibitive interference in your plans.
The second reason is that you need A-10s or something just as lethal against ground targets. The Su-25s are in that vein but don't carry the killing payload the A-10 does. Of course, the USAF Idiots-In-Command want to get rid of the A-10 without replacing it with something as good or better. Fortunately, the IIC have been stymied for a while. But if WERE going to get rid of them, I'm sure Ukraine would be happy to have them along with some Flying Tiger Yankee pilots. Still, they really can only operate effectively for a long period in an air supremacy environment.