Dynamically Linked said:

Seriously, most moral philosophies are against cheating, stealing, murdering, etc. I think it's safe to guess that there would be more cheating, stealing, and murdering in the world if everyone became absolutely convinced that none of these moral philosophies are valid.

That's not a safe guess at all. And in fact, is likely wrong.

You observe that (most?) moral philosophies suggest your list of sins are "wrong". But then you guess that people tend not to do these things because the moral philosophies say they are wrong.

There's another alternative. It could be that human behavior is generally constrained by something else (e.g. utility maximization), and it is this far more fundamental force which prevents much "immoral" sinning, and that explicit "moral philosophies" are actual constrained by observed human behavior.

In other words, you've reversed cause and effect.

(Thus: the moral philosophies are not valid, but the behavior constraints are still rational nonetheless.)