Because if they do, then if the company goes bust they lose their job and their savings on the same day. See Enron as an example where employees were heavily invested in Enron.
And the company doesn't even need to go bust - a downturn in the stock price leads to some cutbacks and redundancies, and you're the unlucky one...
The first rule of investment is diversification, and keeping your salary separate to your savings seems like a good start.
By all means buy options, and sell for a quick bonus, then take the money elsewhere. If you can't sell the stock (company not yet IPO) then treat money spent on options as "lost"... In 99% of cases it will be.
Also, the idea here is not that it's a great investment (although it usually is), but that it transfers power to experienced employees. Imagine if you and your coworkers owned enough company shares so that you could veto the CEO together.
My point I guess is that there's no materially different upside to owning shares where you work, or just shares outside. And shares outside diversify your income stream.
If there is a material difference in terms of power, well then I guess that's different.
Perform better towards what end?
This is crucial. Many bootstrapped companies don't offer much stock ownership from employees. Yes, they can pay a good salary, but these employees are laying down the business brick by brick, but never see a dime of the upside. Mailchimp comes to mind. I'm sure there are others.
This basically leaves stock ownership in VC-backed startups as a way to get rich quick (albeit with low odds): https://topstartups.io/startup-salary-equity-database/
In fact, Mittelstands will probably perform even better if they can figure out how to attract the kinds of talent well-funded startups do. And from there, it'll be a virtuous cycle.