Compare average programmer's salary Chicago vs. Mountain View...
A big company like a FAANG will shift the local market when they open an office.
There are plenty of competent people all over the world that can’t move easily for whatever reason - usually family.
Yeah, it's a more generous explanation that they want to tap into the Chicago area talent pool. It's possibly true, a new office like this was no doubt planned years ago.
Last time I checked, Chicago was not in the Bay Area.