* there's 7% extra that is paid as a lump sum when you change employers, and 33% paid into a retirement account (vs. perhaps 20% for a 401k; employer/employee proportion of 25-75 vs. 50-50 for 401k matching)
* you don't enter work with any debt, because university is paid by the state if you're poor. I have seen directors from America (easily $200k salary) saying they can't afford a vacation in Europe because they have two sons/daughters in grad school.
* you don't have to pay for healthcare (and taxes are comparable for lower income tax brackets). If you have a child, pregnancy+delivery costs €0.00.
* nowhere in Italy are you going to have a real estate market as crazy as Silicon Valley
* childcare will cost 150-500 euros a month depending on age vs 1000-3000$ in the US
Putting everything together, maybe you're paid twice as much. Go to Germany or Finland and there's even less difference.
Besides one can't really compare IT jobs in UK and USA. In UK they are generally at lower end of pay and reputation compare to US.