Only because we have been boiled to be ok with it.
It used to just be x-ray and a bag scan. Now it is a backscatter plus a pat-down if required plus a bag scan. If you get flagged on either, your hands get swabbed down as well. The tech and number of searches is only going to increase over time as more contracts get in on the TSA grift.
I have zero doubts at some point they will also begin domestic device scans ("just plug the handy USB-C into your device and hang tight for a few while we ~~download all its contents~~ search for terrorism!) like they already apparently do for international flights under the guise of national security and making sure you aren't plotting something.
The self-service TSA booths are to save money on their minimum wage security staff, not to relax security. It increases the amount of surveillance they can pack in by putting you individually through a chokepoint.
Think of what you could do if you could lock people into a booth by themselves as they do tasks to be let out. I am being a bit hyperbolic but singling out a person in a potentially enclosed box opens the door massively to techniques such as strip searches (you don't even have to move rooms, how convenient, thanks TSA!) and illegal searches (ugh, really, just let us do it. It'll take 3 minutes and besides, no one will ever believe you that we asked because it's just us here), etc.
Then you get into the fun tech that is opened up by having a person isolated such as bomb detection through analyzing residue in the air, far more cameras that can be analyzed by AI/ML ("your left eye twitched 27% more than average: TERRORIST") and analysts at scale, mandatory facial recognition and other biometrics collection (a big issue with facial rec is making sure no one interferes, it's very easy to detect a face when there is only one in the room and you can have the perfect solid color background)