Two reasons. First, because there's absolutely
no serious political mandate to do otherwise. Once you get past the abstract level where most people agree that deficits are kinda bad and something should be done about them, you discover they are entirely full of shit when asked about specifics and only want cuts to tiny bits of spending they ideologically oppose.
Americans entirely full of shit on spending. (headline changed for accuracy)
Second, for the reasons I mentioned earlier... because we can. What about our habits in the last (pick a time period, 25, 50, 75, 100) years would lead you to believe that
anything other than an immovable brick wall of reality would make us stop?
Yes, tax cuts are the conservative version of it. It's
spending but it's spending they like.