Because from the restaurant's POV it seems like free money. If no one tips, they perceive no harm. If someone tips, then free money.

It's also more money to the company providing the payment system so they are incentivized to tell every restaurant to turn on tips.

I hate it. I don't know how to fight it except to try to start a movement big enough that everyone who goes to such a restaurant gives it 1 star and not not return, until it becomes unacceptable for business to do it. Like many things that is unlikely to happen without some major awareness campaign.