398 jobs are in Palo Alto, CA, which is the heart of Silicon Valley. California prohibits non-compete agreements, so it is much easier to hire in CA. Silicon Valley has a huge number of experienced software engineers / programmers. There are lots of big technical companies with employees looking for greener pastures. Most of the self-driving car technology is being developed in SV.
If you want to grow big, with the best talent, your tech lab must be in Silicon Valley. Both Microsoft and Amazon have large facilities there.
Rivian is officially headquartered in Plymouth, MI.