Because the majority of people don't deserve to make money off the little knowledge that they have. Magic is for entertainment. Just because you can do magic, does not mean you should be paid.
That's a bit cynical. Acting is for entertainment, playing music is for entertainment, dancing is for entertainment, being funny, of course, is for entertainment. They're all things that people are willing to pay to see. Why not take advantage of that?
People are willing to pay to see these things if
they're done well. But most of the paid performers in these disciplines didn't start out as paid performers; they worked their way up in some fashion.
Actors take acting classes and perform in school/community theatre. Dancers take dance lessons and give recitals, as do many musicians. Comedians try out their sets on preordained open mic nights at comedy clubs (some even apprentice other, more experienced comedians first.)
If someone were to memorize Hamlet's soliloquy then show up at an audition expecting to be referred to and paid as a professional actor they would quickly be shown the door. Unfortunately, this is how many beginner magicians are approaching restaurant magic. They learn a few tricks, maybe even show them to a few people, then figure, Hey, I'm ready! Best case scenario: the restaurant manager sees through this and says "no thanks." Worst case: they get the job, perform horribly, and convince the manager that magic has no place in a restaurant.
Restaurant magic is not a beginner's launchpad; it's a profession to work up to. There are plenty of ways to start doing magic outside of restaurants: family shows, volunteering in hospitals and senior citizen centers, etc. There are also "regular" jobs that one can take to further prepare for a restaurant gig---being a waiter and/or working a retail job come to mind. Not to mention, there are plenty of resources put out that teach both the basic and finer points of restaurant magic. Someone who comes onto these boards and asks when to approach a table and how to structure a routine is not ready to be a paid performer. Such people can come to be ready, but they must be willing to pay their dues to learn their lessons.
People can ignore bad actors, musicians, comedians, etc. because they have plenty of access to good ones in the mainstream media. Few people encounter magicians on a daily basis, and so their opinion of one can affect their opinion on magic as a whole. It is the magician who recognizes this and takes the proper steps to become a paid performer that shows respect for his fellow magicians, and that is the kind of performer that will in turn be shown respect by his peers.