Do you have a source that says the average experienced engineer in Silicon Valley makes $400k/year?
[0] https://www.levels.fyi/?compare=Uber,Google,Microsoft&track=...
But then you are talking about a small minority of even SV salaries, and a tiny minority overall.
The real issue as noted elsewhere is that the distribution is at least bimodal, and many conversations get derailed by confusion around this fact.