Because they are a reasonably long lived organization with at least some rules and rituals that can be used as a base for theories. If they didn’t exist, a different organization would be used as the seed, or a fictional one would be made up. Unless you can think of a number of other organizations that for some reason aren’t picked on the same way? Church can’t be used quite the same, because they are to mainstream and inclusive, though secret societies within churches often get the same treatment.

That’s their strategy, let some laymen in to look around, show them some fake ‘secret’ rims to show they aren’t really that special, while the clevery hidden real secret doors are quietly moved as you leave and enter each room. You end up being just one boring anecdote on the Internet, but over centuries it adds up to hundreds of ‘eh’ accounts to hide the real story… It’s brilliant!

Basically nothing