Considering a shit ton of employees in SV are working on their own projects in their spare time, start their own companies, and moonlight - I don't think this is as common in contracts as you're making it out to be.
No, a great deal of that is explicitly prohibited.
If you work for Google, they own anything you develop on the side. They're actually nice in that they have a review process where they will give you your rights back if they decide it isn't competitive with any of their lines of business.
A lot of other companies don't even provide that. If you start a business on the side, they own all of its IP. Period.
This is extremely common. Both with large corporations as well as with startups. You just might not be aware of it.
I’ve done startups myself and it’s not common in any contracts I’ve signed nor have my coworkers. If it was, we all wouldn’t be able to start our own companies.
The example is in the headline.