They are too large to give new employees significant stock options, yet too small to pay FAANG-competititve rates. So you're starting with less salary and no stock. If you now have an egomaniac boss who thinks his own personal brilliance is always the cause for the company's got fate, then there's a good chance he will just take credit for your invention and your bonus is gone.

And in slightly larger companies, chances are someone in the management chain will take credit for it and get the bonus. There's a reason why in most companies, you can never talk directly to the upper management: Middle management has strong financial incentives to prevent that.