Because you're thinking about it too hard or otherwise unaware of the inner turnings of the system.
Where it matters, servers are almost their own industry, swapping gigs at this or that restaurant to work with this or that regional manager. The restaurant gets their money from the bill, the server gets their money direct from the customer they helped.
My server goes to a different restaurant -- I go there and keep getting awesome service.
This type of arrangement gets really close to a guildhouse/coop more than a franchise.
In places with less stable relationships (tourist restaurants), asking for tips might be a predatory thing -- or you can see it as paying it forward: "hey person working to get people their demands in a moments notice, I hope this tenner will help you hold that smile in the face of another consumerist monster. Thanks for being a human."
