One automated human verification process that I've seen in the wild:
The Bumble dating app has you verify accounts by taking a photo of yourself making a pose.
Your account gets verified almost instantly. I'm not sure how it works, presumably some algorithm compares the verification photo to your profile photos. Plus there are probably other data that go into verification beyond the photo.
I'm curious how well it would stand up to AI generated profile pictures and a deepfaked verification photo.
Another verification process I've seen:
When you apply for a loan online, you are challenged to answer questions such as: select from this list of addresses the ones that you lived at, and select from this list of cars that you once used as collateral for a loan.
My point being, it's not impossible, but you're right that it won't be cheap. And not everyone on Twitter will be able to take part in automated verification for one reason or another.
Plus Twitter depends on free accounts to have enough people at the party. Or at least in the growth days that was important. Maybe it's not a priority anymore.