That's the excuse I always hear
It's not an excuse, it's the truth. They'd have to completely create a spectating system from scratch which in your suggestion ONLY adds the ability to join new games after you've died. If you ask me that's a big waste of developer time.
I'm not saying they should revamp it now, immediately. If Cubecraft would ever do a big update of tiny fixes and additions, this should -in my opinion- be on their list!
I'm not sure what you didn't understand about the completely creating a new spectating system from scratch? That doesn't just seem like a tiny fix/addition.
It would probably be way easier for them to just add a message in the chat after you've died which you can click to join a new game, ranked players would still be able to choose maps if they wanted.