It's a combination of things, but it's similar to why online stores advertise a price, and then add charges at checkout (like shipping, or credit card charge, processing fee, etc).

You entice the customer with the lower price, and once they have committed, you can hit them with another charge or fee without as much ramification.

Restaurant customers can be price sensitive, so tipping is a way to advertise a lower price that isn't actually real. Also, it's pretty ubiquitous, so unless you heavily advertise a no-tipping policy like Kroger delivery, most people will assume tipping is required, and factor that into your now higher price.