Because the only winning move it not to play. This is Europe's war. Not sure why the US is involved at all. It's not like Ukraine has oil or a NATO partner.
Today the US has two strategic enemies - Russia and China - and two strategic partners, Nato in Europe and everyone in pacific except China.
The US can spend peanuts - it really isn't a lot of money in US defence terms - backing Ukraine and using Ukrainian casualties to defeat it's strategic enemy, Russia, whilst making it's other strategic enemy, China, fear it.
Or it can waver and show it's no longer the leader of the democratic world and make all it's allies in Europe and Asia not believe in it.
My big fear is that it is empowering China to dare to have it's go at Taiwan in a couple of years.
A few more quotes by Trump might change that though.
It certainly seems that the US is unsure whether it wants this role. The Congress is putting US credibility at huge risk right now.
Nevertheless, if the US abdicates its leadership, the free world will shrink. Even democracies have domestic enemies and all of these will be encouraged to push autocracy as an alternative to the messy parliamentary system.
When viewed by how they act? Unquestionably.
Europe is probably uncomfortable/ashamed by how dependent they are on the US for maintaining the western-centric global power axis. But on the same hand are unwilling to make the sacrifices their societies would need to in order to pick up the slack. Especially now that European economies tend to be in a slump.