Because that is how the staff is compensated. The employer expects the customers/members to tip and so they pay the employee proportionally less. I don't agree with this system but it's what we're stuck with in the US.

If I had my druthers I'd start with tip money on the table and add/remove as appropriate duing my time at the establishment leaving the pile of money on the table/bar at the end. That would avoid any miscommunication between the server and the customer and allow poor service to be improved upon.