What are developers waiting for?
Heh, you've got it wrong. In-wallet POS was the first kind of POS. The history shows it was a failure, hence the developers moved on for something they've considered it could be better.
I remember
MagiCoin had in-wallet POS (and also CPU mining). I don't know how active/alive it still is, still it can be something to read if you have the time and mood. Basically it started good, but the price kept falling, the volumes kept also falling, then the bigger exchanges delisted them, then the prices have fallen even more...
It was a coin I liked. It was also a coin that made me understand that in-wallet POS is not a good idea (and neither CPU mining).