Because they’d rather depend on an ally than an openly hostile state. Because the US would massively subsidize LNG exports to EU in order to support their allies?
That's the assumption, but it doesn't follow natural economic laws.... where price ultimately prevails.
One only needs to look at how global oil prices were weaponized, in essence Persian-gulf oil producers openly conspired to sell barrels of oil cheaply to deprive their South American oil exporting competitors from realizing higher gains. And... one only needs to look to Argentina to see how that played out. The point is, energy importers prioritize lower price, and energy exporters are willing to maintain lower prices to protect their cartel or monopoly.
Russia has provided affordable cheap natural gas as a strategic goal to create an effective monopoly in Europe. Not recognizing this monopoly is a form of denial or rationalization, and isn't very helpful. Europe simply cannot cut away the lower energy prices, as that has deep macro economic consequences.
Spending that money, or a bit more, in the US, at least I’m not funding an openly hostile regime. Some would eve argue you could reduce your defence spending by some amount $Y.