https://smartasset.com/taxes/california-tax-calculator#
and
https://listentotaxman.com/?ingr=100000
Someone on £100k a year in the UK will keep £68,561
Someone on £100k a year in California will keep £69,291.73
That doesn't feel a massive difference.
Even when in UK is the same, you still have to pay the TV-tax, VAT 19%, fuel prices are high because of gigantic taxes, all of those do not exist in USA.
Also in Germany public health is getting more expensive (there was an increase of 1% this year) and worse (although still good). Also education is getting more expensive slowly.
Salary difference isn't because of tax (employer NI contributions in the UK are high - £12,500 for £100k, but employer healthcare is high in the US)
The assertion was that employee taxes were high in europe. Now they may be in mainland, but in the UK that's not particularly true, not at 100k range anyway.
On the California median salary of $67k, in the US you keep $51,915, in the UK you keep $51,741, practically the same.
At $500k, in California you'd keep $287k, in the UK $279k, so 3% difference there at the 8-times median CA wage, I'll admit that.
At that salary though the employer contribution to tax in the UK is significantly higher than healthcare contribution in the US, so sure, probably around 10% difference in total.
There's basically no difference in income taxes between the UK and US, that doesn't explain the massive salary difference.
EDIT: median salary in SF: https://www.levels.fyi/t/software-engineer/locations/san-fra... would be >350k USD.