Unlike most posters on HN I cannot speak for the whole industry, since I actually had been working there for a couple decades so I can speak only about my motivation. First of all it's interesting. All I do is solve interesting math problems and get paid for that. Then there is money: I get paid a bonus based on the game's sales. And some vanity too - there are games around whose authors are already dead yet they are remembered because of these games and their creations left a significant impact on millions of people.