Right? It's pretty disappointing, the Edge could definitely fit one or two AAA batteries, and you can definitely engineer something to be waterproof and have replaceable batteries. That's not new tech exactly. The entire Edge 2 (and I assume most toys like it) is coated in silicone, so I don't think there's an elegant way of opening it without breaking the silicone. Not sure if there's many teardowns of these things.
I used to charge the Edge after using it, but after coming back to it with very low charge sometimes I've elected to just charge it beforehand. Granted I don't use it that often, so I really can't speak for its longevity. When I do use it more frequently it has enough for a really lengthy session, or two more normal ones.