When Microwaves first came out they were huge but with a tiny cooking area and they were not affordable, and were mostly in business kitchens and a few restaurants. I would love a freeze dryer but the cheapest I have seen is $1600 and they only have 4 small trays.
It depends, some things, like freeze dried fruit may not necessarily need to be rehydrated.
For things that need to be rehydrated, you may not need as much water to rehydrate it to be edible as would be in the regular ingredients. Hypothetically if you were to make soup from scratch, you’d lose some of the water to evaporation as you cook it. If you were to premake and dehydrate soup, it wouldn’t need to be cooked as long or to as high of a temperature - everything is already cooked you just need to rehydrate it and warm it up to your liking, no need to get it up to a boil and simmer it for however many minutes or hours so less is lost to evaporation.
And depending on the area you’re backpacking in, you’re probably going to be refilling you water from streams and such several times along the way so you can plan around that. In the areas I normally backpack, you’re probably going to cross over or hike along a few different streams every day, running out of water isn’t a major concern.
One time in particular comes to mind, where I did have to plan around having enough water to cook my meal. Normally we plan on our lunch being cold- jerky, trail mix, etc. and we do a freeze dried meal or something similar for dinner that requires water. Around lunch time we were by a stream, and looking at our map the area we were planning to camp for the night wouldn’t be near a water source (pretty much at the very top of a mountain) so we decided we’d have our hot meal for lunch so we could refill our water to make sure we’d have enough to last us until we were able to refill later the next day.
It kind of sucked though, as we were getting closer to our campsite, the temperature started dropping, and a thick fog rolled in. By the time we made camp, we were all kind of cold, everything was damp, and we were generally pretty miserable, and we didn’t even have a hot meal to look forward to. So we pretty much just scarfed down whatever jerky or crackers or whatever we had and went right to bed. The next day though, everything had cleared up, and when we made our way to the summit to enjoy the view. We looked down into the valley below us and we saw a cloud, and we realized that the fog from the night before wasn’t just fog, it was a cloud passing over the mountain, and we hiked through it, so that was pretty cool.
But the next time you go mattress shopping and the salesperson is telling you “it’s like sleeping on a cloud” run away, clouds suck and don’t make for good sleep.