Because the US is the primary enforcer of the world order that mandates, broadly, that everyone can trade with no fear of attacks on their shipping, and that we don't steal each other's land any more.
When the US gives up on that role, the world is going to change drastically, and not for the better.
Plus that conflict is quite complicated, predates WW2 and the imposition of this system, and the US does not support the settlers.
It's not that complicated, actually. The US is technically not a fan of the settlements, but its criticisms have been muted to the point of silence for several years now.
Meanwhile, what matters is what happens on the ground. As long as Israel keeps doing the settlements dance, and US military aid keeps flowing with no restrictions, and it does not take aggressive action against the settlers' base of support within the US itself, then de facto "the US supports the settlements", and that's pretty much all there is to it.
On top of this, other extremely regrettable actions it has taken (such as endorsing move of the capital to Jerusalem) signal tacit support of the Greater Israel project generally (which the settlements are the primary vanguard of).
That's not an invasion in the classic nation-state sense since the Palestinians have never had a country.
What matters is that it's both theft and plunder on a grand scale, and ethnic cleansing in the classic mold. Mincing words does not change this fact.
