Well, MREs are shelf stable and the retort pouch packaging is very robust. They keep for potentially well over a decade, and can last for several years even in extreme temps (like 100+ F for a couple years). They're designed to fuel soldiers so they're fairly high in calories. And of course they require no preparation at all. They can be heated (and usually a chemical heater is included) but they can be eaten cold right out of the pouch. Certainly in some situations the lack of prep required can be a godsend, especially when you're already exhausted or you find yourself in conditions where cooking is impractical.

Note that I'm not starting an MRE fan club. Just pointing out why they're popular with outdoors-folks.