Giving someone a tip 'personally' doesn't always ensure that person will get to keep that tip.
Some places have a mandatory tip pool that is collected and divided among more than just the servers to supplement their pay.
There's always the other despicable practice of 'kicking back to the house'. Some servers pay 1, 2, 3, or more percent of their SALES back to the house.
So if a server sells a thousand dollars in food and drink during their shift, the house might get 10, 20, or 30 dollars whether the server gets tipped that much or not.
Usually they do of course, but it amounts to paying to keep your job.