I think you're wrong. Perez, through his Mexican sponsors (so, indirectly) would most definitely pay to drive the MB.
Key words those...."through his Mexican sponsors". So, I'm correct. If you're a true racing fan, what they earn has nothing to do with the racing. What has what they earn have to do with "sexiness"? (Besides, what has sexiness to do with racing?)
I don't really care because it's not my money. But for the sake of intellectual practice, I guess it happens because F1 has become a self-protecting closed world - it has of course advantages, but the drawback is the lack of connection with reality. F1 is not the only environment where it happens IMHO.
When you have a $400 million program, you not only want a fast driver, but one who won't be crashing your car and throwing away the effort for a race. For example, Kvyat was very fast when he arrived on scene, but crashed into everybody - the team threw him out of the car. It's amazing to see that he's back, but his drive craft is very much improved. The top drivers are fast, cool, consistent, and don't make mistakes. Not making mistakes means being able to concentrate in demanding conditions for the full race. This is where Vettel is failing - he just doesn't seem to be able to concentrate for the whole race while being fast. Verstappen is exceptional in that he pushes harder, maybe bangs around a little bit, but doesn't crash. Leclerc is learning that from him. The top drivers are worth their high salaries.