What most people don't realize is everything there is around the capsules. Usually, when you enter the hotel, you start by going to the locker room. There you put in all the stuff you won't need for the night, that is, essentially everything. You may even get a yukata robe, a towel and some toiletries, so it can be literally everything, including your clothes.
Then you can take advantage of the amities and relax. You may find showers, hot baths, massage chairs, etc... Only when it is time to sleep you go to your capsule, which is essentially like a bunk bed with privacy, maybe with a bit of equipment like a TV, alarm clock, charging plug...
Now, I probably wouldn't stay in a capsule hotel for 3 weeks, but I could definitely spend 3 weeks backpacking, stopping in capsule hotels for the night. It is not really designed as a place to stay, more like a place to spend the night with no preparation. Also, while it is cheaper than regular hotels, it is not that cheap, and if you really want cheap, there are other options.