Why do us need to pay for the taxi driver?

If we use the services we should pay for it. There's no free lunch.
Taxi drivers use the taxi to provide services in exchange of money(fare) from customers.

First of all, your question should be "Why do we need to pay the taxi driver?"

When you flag down and board the taxi, you have engaged into a contract with the taxi driver. A contract has an offer and an acceptance. The taxi driver has offered you a service in return of money, you have offered money in return of his service, which is to get to your destination.

英文(Because we have taken the taxi, we must pay a driver.)
中文(因為我們已經乘計程車，我們必須付款給司機 )