Sometimes because they’ve had enough of it and want to do something else with their time - be that working on a new idea or changing careers or just working for someone else and no longer having to be the one who lies awake at night wondering whether revenue will cover salary this month.
Bootstrapped doesn’t by definition mean profitable. Some founders are using their own savings or bank loans or even credit cards as runway.
Also, even bootstrapped businesses that are doing better than breaking even may not be making enough of a profit that the founders are doing better than they would with a corporate salary.