Because it works, quite well actually. It isn't that hard or expensive. And it's convenient. Why push for the old stuff? There's absolutely nothing fun about having to queue for some TSA prick for two hours after a transatlantic flight who hates his pointless, miserable life (and rightfully so). All that stuff can be automated these days.
TSA does not do border control, and in fact border control is usually relatively fast compared to being re-screened through security (TSA).
Edit:
It's convenient if you are a digital native, but elderly folks, among others, will not find it easier than a physical passport. The push to require everyone to have a digital device to participate in society is troubling to me.
I guess if that's how you feel about it, more power to you. The day I get away from almost all tech will be a good day. Also I get that TSA sucks but I don't think they deserve the vitriol you're throwing.