What happens if you work another job and invent things there? Do the companies own each other’s stuff?
I suspect employers love restricting your right to have a side hustle so that they absolutely totally own you. If you have no other means of income besides your job they can lean on you pretty hard and there isn't much you can do about it if you like having a roof over your head.
I like to frame it in terms of reputation risk to the company. If they own the contents of people's creative hobbies outside of work then I am happy to put "Copyright $COMPANY" in big letters on my amateur porn website... Surprisingly few companies have thought about it that way.
What does this mean?
Speaking as an employer, what my employees get up to in their own time is their business as long as they get their work done when they're supposed to.
In theory, yes, In practice, nearly every single US employer has this somewhere in their contract automatically by default -- the only way a company doesn't, is if someone explicitly got it removed.
Also, in much of the US, the clause is enforceable, so companies have basically no incentive to remove it.
However I did sign a forced arbitration clause which is equally BS.
You know, you don't have to sign those.
I always cross out those and tell them to send it back when its fixed. Only my first job I was afraid to do this.