Yeah, 100% this. While quite a few people do indeed leave companies because the boss is terrible or they hate the working environment there, a lot of the time it's simply because opportunities for advancement or making more money are limited in said company. A tiny startup or lifestyle business can't exactly pay FAANG level salaries, and there's not much room for a promotion if the company has less than 30 or so employees.

And if their situation changes outside of work, then finding a better paying job can be a necessity regardless of how much they enjoy their current one.