Joe90
96
Can't say for sure but a few theories?
Most people
Rolex is not "cheap" for most people. Having a "normal" job, most people need to save up to buy a Rolex. Combined with Rolex's marketing and armed with the knowledge that it's "easy" to get most of your money back on a Rolex purchase, it's a safe choice for most enthusiasts. They know/think that even if they pay above the odds, they can probably sell their watch given the assumption that demand remains high.
Patek is the higher end version of the same story.
"Most people" might be interested in other brands if they could easily afford them - and afford to lose money on a sale.
Wealthy people
Then there are the population that can fairly comfortably purchase a Rolex (or a Patek) on a whim. They recognise the cachet that both brands have and they purchase several recognised and revered watches as part of their collection. Possibly jewelled models or ones in less ubiquitous precious metals too.
They could purchase ALS, Philippe Dufour or Kari Voutilainen, etc (and they might still do) but it draws blank looks from the general public, so they still need a Rolex or Patek in their lot.
Lastly
Rolex (and to a lesser extent PP - unless we're talking exclusively about the Nautilus and Aquanaut) are fairly practical one-watch pieces.
Reliable, serviceable, sufficiently water resistant, etc.
Perhaps it's a bit of a "disease" but it's not difficult to understand why they're so popular.