Why is it more profitable for a node with more than 51% computation power to abide by the rules in the bitcoin network The Bitcoin whitepaper states - "The incentive may help encourage nodes to stay honest. If a greedy attacker is able to assemble more CPU power than all the honest nodes, he would have to choose between using it to defraud people by stealing back his payments, or using it to generate new coins. He ought to find it more profitable to play by the rules, such rules that favour him with more new coins than everyone else combined, than to undermine the system and the validity of his own wealth."

I don't quite understand as to why? Can someone explain as to what can an attacker gain if he has more than 50% computing power and how it is outweighed by what he can gain if he plays by the rules.

The question What can an attacker with 51% of hash power do? does not seem to answer my question as it does not clearly seem to explain WHY is it more profitable to play by the rules.
An attacker with 51% hash power could perform a double-spend (or several). He could send a large payment to someone and then use his hash power to create a longer chain that spends the same coins but sends them to himself instead, making the first transaction invalid.

It would be against his interest because playing by the rules would net him roughly 1 every 2 block rewards (that is roughly $100,000 every 20 minutes) but if he chose to perform this attack the price of bitcoin would plummet making his reward much less.