For two reasons, one is to incentivise staff to give you the customer a good service as the level of their reward depends on the quality of their service and level of attentiveness to you the customer. Bad waiters get bad tips and good waiters get good tips. Performance related pay exists in many spectrums.
And the other is to do with tax; if the restaurant wants to pay their staff £1 more on their take home pay they need to charge you enough to cover the PAYE, Employees NI, Employers NI. That would cost them £1.77 in total. So they need to charge you more to do that. By charging you more they have to pay VAT on their takings before having enough to pay the increased wages, Paye & NI. So that £1.77 becomes £2.08.
It is wrong that many resuarants use the service charge to top up wages to the basic minimum. Refuse to pay the service charge and they can’t do that. Just tip the waiter, in cash, directly