I hope you trust me now that it is correct what I am describing and that I understand well that the cache needs to be rebuilt IF any changes within the sceneries are made as I understand what GSX normally does. But obviously there is something not normal with my installation.
It's entirely normal Couatl would restart when returning from the main menu, because this has been found to be the most reliable way to be sure every object created with the previous session would be removed for the next one.
What is not normal, is the cache is being regenerated at each start because, from GSX point of view, it should only happen if the previous cache regeneration failed for some reason, but if it did, it would show in your log, but it doesn't.
So, the only possible explanation is that, for some reason, something in the simulator community folder is changing AS IF you enabled/disabled/added/removed a scenery, triggering a cache rebuild.
Nobody ever reported this problem happening this way ( regeneration each time with no errors ) so, it clearly must be something very unique to your installation that plays with files/folders, since GSX checks a folder last modification date to know if changes have happened, perhaps something like an antivirus that doesn't do anything other than changing that data, just to flag it has checked that folder.
However, I don't think it's really worth investigating further, because when we'll release the SU10 Navdata upgrade, which will allow to use GSX on any airports, including the encrypted ones from the Marketplace, the other major benefit is that it won't even use an airport cache anymore, because it will take airport data from Simconnect without having to open the airport file.