This is it in a nutshell. Most of the calorie calculators out there, be they on your Garmin or via a site like Strava, overcalculate calories burned and most people don't eat sensibly following a ride so they don't actually end up burning nearly as much as they think they do. Regardless of the sport someone does, nutrition is key. You have to put the right amount of "smart" calories in on an ongoing basis and not replace too much following a vigorous workout.

The biggest issue, though, is eating right is hard. You ideally should do several small meals in a day and that all takes a lot of dedication and some prep time. In my experience, only really zealous amateur athletes or fitness junkies are willing to put in that effort.