Because Russia has stated when they started this phase of the war that they want NATO to return to its borders before the fall of communism. This means reoccupy

Because Russia has stated when they started this phase of the war that they want NATO to return to its borders before the fall of communism. This means reoccupying all the eastern European countries that are part of NATO. Assuming you believe in honoring the alliance, it's a lot easier to provide Ukraine with ammunition than to let it fall and have to send American troops to fight in Poland/Romania/etc.