Because they have nowhere else to go to compete at the very top level.
Italy is second tier European football these days and has a lot of problems with corruption. Neither Guardiola nor Klopp seem interested at all in going there.
England is out for Klopp (and for Guardiola unless at Man City). So is
Germany (Bayern is good but their league is bush-league; Klopp managed in Germany, Guardiola did Bayern).
That leaves
Spain where Real Madrid doesn't have patience for a great manager with his own style and transfer requirements in terms of player type. So that's out (for Klopp and anyways for Guardiola). Barcelona is an option for Klopp but he doesn't play their style of football (would they even have Klopp is a question; plus there is Guardiola issue there, maybe they may want him back at some point). That leaves places like Atletico or Valencia. And between the two I think that Atletico would be a much better option because Simeone has been doing a decent job with them for a while so they have a decent squad/systems in place. Valencia is nowhere to be seen.
For the likes of Guardiola and
Klopp there is basically nothing else in the whole world football. Guardiola could perhaps take up some lower ranked Spanish side and manage it to some sort of decency. He could do that out of some sort of a pleasure or somethin'. I don't know but I don't think he's a type of person that would do that. Klopp might be pressured by Germans into taking on Bayern but that would not be top level football - just in terms of Champions League which is a less frequent, side competition.