I don't understand it. Almost every 10th or 20th question is about fan or overheating or that the fan is too loud and the speed to big etc.
I think if we did a more realistic survey this would prove a grotesque exaggeration. I monitor this site a lot, and while there are hundreds of questions per day, this most recent overheating question is the first one I've noticed in weeks (of me watching, not real time), so realistically, it is more likely that no more than one in 20,000 questions has to do with overheating. I'd also bet that of those, the majority in the end turn out to be hardware issues that have nothing to do with linux or any other OS.
Just glancing at the "Related" questions in the sidebar, and considering "fan" and "overheating" are in your title, it would seem there have been very few questions of this sort. O_O?
Why there are so many problems with fans in the Linux world?
There aren't. I've been a user for ~15 years, including work as a professional, with dozens of systems and never once personally seen a linux related fan or heating problem. I'd bet if you surveyed linux users generally, 99% of them would say the same thing.