The version of MREs before the current tan bags, the brown bags (Desert Storm era), were freeze dried meals.

They also come with a lot of stuff you don't want to eat for an extended period of time, and a lot of trash waste.

It appears that space was at an extreme premium - and remember, if they rowed, weight matters as well. MREs work in an environment where you have trucks (or planes) dropping off pallets of them on the regular, not where you need to bring 6 months of them along on something you are moving under your own power.

Long distance ultra light hikers and expedition type folks tend to bring freeze dried food (either way they can’t carry enough water so need to figure that out). Vitamins to avoid any weird effects from missing nutrients from ‘found food’ was also a good idea.