Because that is what pretty every US American independently from political affiliation i talked with about this told me. They need to spent their own money on things that is covered via taxation in many parts of Europe. Whether this is good or bad, is a bit a matter of opinion.
I have absolutely no doubt that if you want to maximize personal compensation and benefits, the US is the place to be. Nothing else comes close
But originally this threat was about "Why compete with FAANG trying to pay new grads $300k when you can pay an experienced developer from Europe $100k? I honestly never understood it."
I doubt that a statistically significant amount of new grads get 300k in compensation plus those kind of benefits. I haven't spent a whole time researching but I couldn't find anything backing this up. E.g. it seems that you need to stay at Google 5 years to get 25 days vacation and entery level total compensation at FAANG being more in the 150k - 200k range.
On the other hand as an experienced developer making more than a 100k in just salary plus at least these benefits is not that hard in Europe if you are willing to move or find a remote job.
From all I heard or read, outside FAANG, the financial industry, and certain hotspot, the compensation and benefits within the US go down quite considerably and the difference to Europe is still big, but less exorbitant.
So sure, if you come in with full FAANG (many / all? of which have had engineering teams in Europe for a long time) style compensation packages plus benefits you will be able to attract many Europeans, especially if they don't have to move. But then we are quickly back to square one: Why compete with FAANG?
And again, I'd argue a big part of the reason why US companies can offer these much higher compensation package to positions like software engineers is because they save it on lower end jobs and there are fewer regulations for the common good in place that would cost them money. For some, this makes US companies a non-starter independent from the individual offer.