It has nothing to do with Silicon Valley. The geography is meaningless. The only thing that matters is staying ahead of the curve. It's always going to be hard for GM to do that, because it's focus is always going to be on incremental improvements of the same way of doing things. The "years of design and manufacturing experience" you mention.
Look at their $1B acquisition in Cruise Automation. Like you said, GM is not a power company, its' a car company. Its' business is cars. GM should have developed self-driving tech internally, and not had to buy it. Cars is GM's job. But GM missed that boat entirely, because everyone in GM is focused on incremental improvements on manufacturing.
Even your comment mentions that "most people don't care about EV right now." EV is not a big focus for GM. Sure it has the Chevy Bolt, but that only began development 10 years after Tesla started. GM does not care about EVs right now. It's focus is still on manufacturing traditional cars, and will continue to be this way until someone else forces them to change.