

Because the industrial base for producing critical things like ammunition is nearly nonexistent. Despite USA and European arms support Ukraine has been permanently shell-starved for the entire course of the war. Three years later, even after spinning up some new production, Ukraine’s allies still don’t make enough shells to get anywhere close to 1:1 with what the Russians fire at them (and that was before North Korea started supplying the Russians)
The invasion of Ukraine has made it crystal clear that Europe’s military industrial base is utterly incapable of responding to an actual peer conflict on their own soil, let alone providing a deterrent to wars of expansion outside of it. It would be foolish not to be investing in sovereign military capability in today’s world.
It’s also just prudence when it comes to the allegiance. If America is going to let a much smaller, less powerful ally get invaded and their idea of a “deal” to end the conflict is to cut off all aid to the ally and give the invaders everything they want, then the allegiance amounts to toilet paper. There’s no sense breaking your back to maintain a relationship with an ally who is going to give you nothing but dishonor and cowardice in your time of need.