Because some people tip others as a thank you for the service provided, not as an incentive to create your own personal indebted servant starting at the start of your vacation.
There are some people in the world who donate anonymously to charities also. There is one person who will walk by the Salvation Army kettle during Christmas and drop a Double Eagle gold piece in it while nobody is looking and there are others who will make a big show of "everyone, look at me, watch while I make my donation, can everybody see me?" Some people are self-less so people are obviously not.
I am changing my stance on not feeling sorry for taxi drivers on Cozumel, I truly do feel sorry for them having to provide service to cheap bastard tourists who have entire multiple point rationalizations prepared for denying some poor slob a buck. Don't you think you should seriously consider if a couple of bucks makes such a huge financial difference to you on vacation, maybe you can't afford to go on vacation?