The only way a 51% attack allows you to steal anything is by reverting a transaction you made that sends something from you to me, and immediately replacing it with a transaction that sends the same thing from you to yourself.

This is only useful if you convinced me to give you something in return for the money you paid me in between you sending the original transaction and subsequently reverting it. Once all is said and done, even if I can't prove it (for some definition of proving it), I still _know_ you stole from me, which has its own consequences.

51% attacks _do_, however, allow you to wreak havoc on the chain state, which can sometimes be an attack worth performing.