There is a wisdom of crowds at work, especially when it comes to huge decisions such as immigration - no country offering worse prospects is ever the target of large scale immigration.
Also, citizens of countries with fickle economies and high inflation could benefit from having an income in a more stable currency, especially if they are breadwinners for their extended family, in which case they send back money they earn abroad.
Does being a homemaker as a result of moving countries count as "worse job"? Maybe I'm living in a bubble but I know many couples where one spouse got a job in another country and the other became a homemaker (most common) or got a worse job. There as a huge amount of skill sets that do not transit well across countries/cultures/continents. So much that I'd argue it is the exception when your skill set transitions well (such as computers, sciences).
Yes, it is possible for those persons to invest time and money (go to college, form new network, learn the language and culture) to eventually get back to their original career but by the time they manage to get there they already lost years of their career so automatically they are behind when compared to others of the same age and talent in the same career or if they wouldn't have changed countries. And to be honest, people are only getting older, not younger, so having to start from scratch when the best outlook for that is being able to be were you ware X years ago in your country is not a very attractive proposition. Combined with the SO who moved for the job earning enough for both it results in a lot of homemakers.