The tax brackets in the US tend to be much progressive though, so for people earning lower salaries their taxes in the US are much lower.
At $600k, it's 37% effective tax rate (again, with zero deductions).
You have to make $817k, with zero deductions, before you'll pay 40% effective tax rate in California.
None of this includes sales tax, which is 10-11% depending on municipality in California. Europe's lowest VAT is 17% in Luxembourg.
It is not my impression that "a lot of people" in tech are making that much, even by the extreme standards of the Bay Area these days.
So yeah it's not the majority of people in the Bay Area paying 40% but if both spouses are working at big tech companies and they're more than a few years into their career then they will likely be somewhere in this range and paying close to 40%
A single earner (again, with the unusual state of no deductions) has to make $690k to reach 40%.
For a dual-high-income earner households I concede 40% effective isn't going to be uncommon.