The CIA has acted on behalf of business going back to the 50's, the 40's even with the OSS to a degree. Anyway, our '53 coup in Iran was to help the UK and their petroleum interests. Fast forward to '56 I think and they helped with a coup in Guatemala, ostensibly to counter Communist expansion, but really for the United Fruit Company (Chiquita today, I think). Cuba? In bed with the mafia. I'm sure none of the latter had anything to do with Kennedy...
Southeast Asia, too easy.
In the 80's we have cocaine.
But why? Greed and arrogance. No doubt some skim is occurring at home and abroad. I think another reason is simple "extra funds to keep doing illegal shit." Illegal programs and whatever, you can fund those if the money doesn't exist. "No" money, no oversight, and you can fund running guns and coke off the books...or on the books, but partially paid for by your slush fund. Need 2 million, but you can only have 1.7? No problem.
I really think it is that simple: greed and funding illegal activity (I include bribes and payouts here).