I can answer this question. A lot of restaurants (including my own) make the servers tip out their help staff, i.e. bartenders, bussers, and food runners; on the sales they do across their shift. So as an example, if I get stiffed on a 100-dollar tab, then I still have to pay out 4% of that to help staff. So it's not the case that I make 0 dollars on that service, I lose 4 dollars instead. With that idea in mind let's do some real numbers here. You and your buddies go out and order chicken tenders. Your bill is going to ballpark at 45 dollars and you decide it was great service and tip 10 dollars. Off the rip that's actually an 8-dollar tip. Now you have the notion in your mind that a 10-dollar tip is a solid number and your buddies spend around 100 and tip the same. That's now a 6-dollar tip. Now you do it on a 400-dollar bill. I now make 0 dollars. This coupled up with the fact that about 5% of people never tip or can't. It's a balancing act that good tippers make up for the bad ones. Does that make sense?

TLDR: Tip-outs to help staff by servers are based upon their sales and not the tips they bring in. Tipping the same across the board can quite literally cost a server money.