incredibly narrow world view

I mean as someone who has experienced both the US and Europe I would say this applies to most everyone posting here.

And if we're going to sling insults then I'll say it's hilariously amusing how clearly fragile the egos of the people posting here are.

but some sources state that the bare minimum salary to be happy in places like Seattle, where these engineers need to be located, is around $117k/year.

Those sources also state that worldwide the number to be minimally happy is $95k. I'm going to assume Europe is at least at the global average and likely a bit above since the US on average is at $105k. So if you go by those studies then the new grad in Seattle making $120k is minimally happy while the seasoned engineer in Germany making $90k is not. Not that I agree but if we're going by the study you mentioned then that's the conclusion.

In the US I can quit a job with 0 days notice. I can also be fired from a job with 0 days notice and no severance.

That makes the job market significantly more fluid. If there is demand for a specific job compensation will go up quickly as there is no artificial buffer on people switching jobs for better pay. Supply and demand is very sensitive to small shifts in either. Companies are also not afraid of paying this compensation since worst case they'll just do some layoffs.

If a US company has some employees in Europe then they still have a benefit from all this so they can pay more than a purely European company. If they need to cut costs they can fire the expensive US employees first and then adjust the Europe comp more slowly. If they need to grow quickly they can do so in the US and then slowly shift to Europe.

edit: The profit margins of US tech companies are also massive as they have relatively little regulation, taxes or bureaucracy. Goggle makes $2 MILLION/employee/year. So no risk of not making record profits by paying an extra $200k.

You're right. But half this conversation is a bunch of people using random US stereotypes and downvoting anyone who says otherwise. Cost of living is fairly irrelevant when you've got an extra $200k/year post-tax to play with. And any company paying that much will give you really nice benefits including fully coverage health insurance and possibly a on-call concierge to help if you have any issues. Being poor in the US is really miserable but I also know people who can't see a doctor in Europe due to waiting lists (or their GP blocking it) and lack of money for private insurance. Neither case matters if you're an engineer. And France has the same rate of homelessness as California so neither has a happy community on average.

45% is the absolute highest tax rate you can currently get we have higher social security Expenditures as well,

So it's higher than 45% in reality (for example I believe at $100k the social contributions would be twice as much as the tax). That 27% for California includes everything. If those taxes are worth it are a separate issue.

No, taxes are basically the same,

Source? Everything I've read indicates Europe has 15-20% more taxes for the same income. The tax rate in California for $100k would be 27% (total) and a 7% sales tax. Europe seems to be in the 45-50% range depending on the country and a roughly 20% VAT. Even the tax rate in California on $400k would be lower than the tax rate in Europe on $100k.

I mean you don't have to constantly job hunt but you have the option of doing so if you want to maximize money. And enough people doing so raises the compensation for everyone even if they don't job hunt constantly.

In my experience managing a large project comes down to having a consistent process/standards and enough experienced engineers in that language. Remove that and every single language leads to abominations.

In my sad experience, you don't but simply try to avoid getting caught in the blast radius of it failing. Someone approved/supported the project and someone approved/supported the sunk cost that already went into it. Those people have more power than you and they will not like looking bad due to you.