I think fans have the wrong idea about athletes in general. They think 100% of the profession actually truly loves what they do. It's the same as any other profession. Obviously Mandzukic is a bad example for this line of thinking but it's a certainty that there are footballers out there who just perform the job because it is literally all they can do in life (moved away from home at young age, poor schooling, no job experience, etc). Might as well collect a paycheck.
For many athletes, sport is just a way of getting out of where you came from. It's not even necessarily a passion but just a means of survival.
And now you have China offering absolutely mind shattering wages in an environment with no pressure at all. You don't have to even train that hard week to week because your opposition is significantly weaker, and you can do whatever you want on the pitch because your coach would never bench you. I mean, depending on the situation, playing football in China can actually be more fun than in any other scenario. You just turn up and play like when you were a kid.
And I'm sure these guys are not living in the parts where you need oxygen masks either. Probably the only time they'll hear Mandarin or Cantonese is when they see fans.
I can see how it is a bit strange for young, promising players to go over there because they've yet to really do the job for some time but any older player, even someone around Mandzukic's age might just be tired of the grind that is European big team football and could easily be persuaded to move.
To be honest, if Hernanes rejected China, I have to give him some respect because I probably would not have done the same. And of course, if Mandzukic is rejecting those wages, he's a hero.