I have to wonder howuch the popularity of all the digital ways to pay is to blame for this. Way back people would leave a few actual bills on the table, it was presumed that the server would get them and all was well.
Now people routinely use debit cards, phones, telepathic toadstool fund deposits, etc to pay for everything. You can use a credit card to buy a soda from a vending machine FFS. As a result it’s simpler to just add it to a single transaction and so everyone wants their token.
I’m always wary of paying a tip at the till though, part of me suspects the staff will never see that, or the owner will claim a portion for just being a swell person, which is not the point of the tip.