Define "middle class Americans"? Hard to give a precise answer otherwise.
A few (other) explanations:
* QoL is hard to be rational about, and is something that (I think) has to be experienced first-hand. European salaries are most probably lower for comparable jobs (they certainly are for high-wage workers) but pack more punch because of better social benefits and, depending on where you live, lower CoL. However it can be hard to convince yourself that 40 k€ in Spain affords you a better QoL than 80k$ in California unless you've experienced both. I mean, 40 is much less than 80, right?
* the "American Dream" is still a big part of the American (and global, for that matter) psyche. I would wager most Americans think the US is a place to emigrate to, not from. I don't know if many middle-class Americans would spontaneously envision emigrating to Europe. This is probably even more true for descendants of recent immigrants: if your family struggled to get to the US, why would you consider leaving?
* Ability to live abroad, in particular mastering a foreign language well enough to live and work in the country
* I'm not sure how well employability travels. You can work as a developer in just about any country but I think this is an exception. A medical diploma does not transfer well between countries (e.g. you probably need to start again as an intern, re-take exams, etc.), neither does a law diploma. More "middle class" jobs will have similar barriers to entry. E.g. if you're a plumber in the US and move to Europe you have to deal with different construction codes, installed base, local habits... Same thing if you're an auto mechanic: different car manufacturers, different regulations, etc.
A few (other) explanations:
* QoL is hard to be rational about, and is something that (I think) has to be experienced first-hand. European salaries are most probably lower for comparable jobs (they certainly are for high-wage workers) but pack more punch because of better social benefits and, depending on where you live, lower CoL. However it can be hard to convince yourself that 40 k€ in Spain affords you a better QoL than 80k$ in California unless you've experienced both. I mean, 40 is much less than 80, right?
* the "American Dream" is still a big part of the American (and global, for that matter) psyche. I would wager most Americans think the US is a place to emigrate to, not from. I don't know if many middle-class Americans would spontaneously envision emigrating to Europe. This is probably even more true for descendants of recent immigrants: if your family struggled to get to the US, why would you consider leaving?
* Ability to live abroad, in particular mastering a foreign language well enough to live and work in the country
* I'm not sure how well employability travels. You can work as a developer in just about any country but I think this is an exception. A medical diploma does not transfer well between countries (e.g. you probably need to start again as an intern, re-take exams, etc.), neither does a law diploma. More "middle class" jobs will have similar barriers to entry. E.g. if you're a plumber in the US and move to Europe you have to deal with different construction codes, installed base, local habits... Same thing if you're an auto mechanic: different car manufacturers, different regulations, etc.