The answer is as simple as it is horrible: It's because for every burned-out, overworked and underpaid game dev, there are two starry-eyed kids who want to realize their dream and create games - and the C-suite knows this.They will replace any veteran dev with someone right out of college as soon as it is convenient
Mind, I am not blaming young people who want to create games. They lack the experience to know they are getting exploited. It's all the cynicism of managers who know no loyalty and only want profits.
And if anyone wonders why every new game somehow manages to be a buggy mess that needs fixing, you have the answer right there too: Because the devs who fixed it the last time got fired and replaced with rookies.