there are already laws on the books that make it illegal to pass any other vehicle in a dangerous manner
The only proof that passing was done "in a dangerous manner" is a
collision. The problem is that motorists have no real information about what is still dangerous without a collision. It's not clear to me that the same distance is appropriate for passing cars and bicycles. Motorists get lots of practice/experience passing other cars but comparatively less passing bicyclists.
The value of a "x foot passing" law is that it gives motorists a clearer picture of what isn't "dangerous". Such a law is nearly unenforceable (but that doesn't mean it has no value).
(Note, too, that there are laws that effectively ban cell-phone use without a specific law that bans cell-phone use.)