Many of these systems are designed such that if you do have the computational power to attack the integrity of the chain, it is actually more profitable to put that power to use for legitimate mining.

Having 51% of the computational power does not let you just make up transactions on your own or transfer funds to your own wallet. What it does allow you to do is pay a counter-party, receive some product from them, and then go back and create a new chain where you paid yourself, instead of the counter-party.

The ethereum whitepaper has a nice explanation of this. https://github.com/ethereum/wiki/wiki/White-Paper