I've signed contracts where they say it's common, and regret it every time. This is a massive red flag.
It doesn't necessarily mean that they plan to screw you. But it certainly means they don't trust you, and it brings all the downsides of lack of trust.
It also means that they're effectively filtering out talent, so your colleagues are unlikely to be very good.
That's what people always say to get you to sign a shitty contract. Consider not taking the job because it sounds like the kind of company that defaults to fucking over the employees in any kind of difficult situation.
I wouldn't sign it. This is not common at all, and while it's probably not enforceable, you don't want to chance it. It's interesting that they have told you that this is common in Silicon Valley because California has restrictions on requiring employees to assign rights to work created on their own time.
This is not standard. If you still want the job, try the following: print it, cross out the parts you don’t like, initial the changes, sign it and send back the scanned pdf. Most of the time HR doesn’t notice or care as long as they can check it off their list.
This clause is abusive unless they're writing you a seven figure check. And companied that are abusive in some ways tend to be abusive in other ways.
If you have any other option I would take the other option. If you have no other options I would redline it and send it back. But whatever you do do not sign that clause.
One could mess with the employer: offer an amendment where they own your work forever, but they pay you, forever, for your time, at whatever rate you set. :)
The employer hasn’t accepted your terms because you haven’t walked away from the deal. Cut off all communication and look elsewhere. If they want you enough to withdraw those nasty clauses, they’ll contact you.