When I visited Palo Alto I found it a very strange experience, my ‘terrain reading’ skills that would lead me to a convenience store and similar affordances in most developed and developing countries did not work at all.
It seemed to me that you could double the population density of that kind of neighborhood by replacing a single family lot with a 8 story tower every other block and filling in an occasional duplex, triplex or small apartment building. Such a neighborhood would be different but still pretty nice by most people’s standards, you can see neighborhoods like that in L.A. or São Paulo.
To be devil’s advocate though I’d say there are larger scale reasons why people might conclude that ‘California is full’. One is that the population density of California overall is about the same as Germany even though most of California is uninhabitable or massive farm fields. Certain regions of the coast are highly populated by U.S. standards even if they are nothing like Hong Kong or Singapore. The linear pattern of dense development means the radio spectrum is utilized like nowhere else in the US such that there are more OTA TVs channels in LA than some cable plans.
California struggles with water and any population addition would require adding more infrastructure of all kinds. It could be more highways, it could be more public transit but it has to be paid for. And the US is a country that spends 10’s of billions to duplicate a train station in NYC or run rail from Bakersfield to Fresno. A reasonable person could very well try to gum up the gears in that machine no matter how they can.
You might say that more population means more economic activity and more tax base but I am not so sure. A lot of it seems driven by inequity and monopoly power. Google and Facebook could not double their revenue by doubling their hiring in California and it is not so plausible that the next big startup in the U.S. would be able to accomplish what they’ve done. There was a time you could dream of selling a company like WhatsApp to Facebook even if going public was not in the cards but the monopolists would hate to have to spend that much to defend their positions and surely try to shape the environment to prevent that. (Now from Genshin Impact to Tik-Tok innovation in social/entertainment is happening in the PRC because that economy is more open to competition.)