They won't get involved until some US troops start dying.
Well, no US troops can die if they aren't deployed in Ukraine
The attack on any NATO member nation is what I think will trigger US to get involved directly, and Putin knows that.
IMO Biden doesn't want to get involved because he wants Russia to do the same in case of a US-China conflict. I hope you remember when there were US carriers going towards China last year and we all know that China wants to take over Taiwan and they showed it once again by saying that Russian move into Ukraine was not an attack. They want to do the same with Taiwan in the near future and this could trigger a move from the US. Biden talked with Putin before the attack on Ukraine and Putin begun to be even more aggressive towards Zelensky after talking with Biden, which suggests they struck some kind of deal.
It's also more to do with their recent change in policies in the Biden administration, like withdrawing troops from Afghanistan. Now, sending troops back to fight in a Ukraine war in less than a year would be ironic.