Culturally most workers in EU markets (I am generalizing but lets exclude Executive, Finance, and Consulting positions) see an offer letter as the end of the recruitment process. People are not confident in negotiating an offer up because they are afraid employers will withdraw or walk-way (Which I have had happen so definitely not impossible). This also means that employers are more likely to see someone negotiating up (for a non-management position) as a 'difficult' IC.
The costs per 'permanent' employee in terms of taxation/contributions is higher than in the US. The taxation on employee income is _way_ higher in most of Europe than in the US. This double stream of government income from employer and employee is needed to finance both the huge slice of government spending (Healthcare, Social Services, Infrastructure) as well as the Pension Systems that at this point are a Ponzi scheme where working taxpayers are paying for retiree income (Privatized Retirement is fairly limited in Europe and mostly optional on top of standard contributions).
You see these two problems dissolve (Culture of content, and high taxation) when looking at the Contractor market, where employers are able to offer far higher rates very similar to the US market when ajusted to COL.
Ultimately lower taxes for the average worker are not gonna happen in Europe because gov deficits would be rampant and nobody is willing to forsake the existing social safety nets.