Because heal-bot is a useless class.
Let me explain. No, let me sum up - That'll be shorter.
Characters without a heal-bot can take on X monsters.
Characters with a heal-bot can take on X * Y monsters.
The game is generally a lot more fun at X * Y monsters.
The game is played at X * Y monsters.
The heal-bot character is there to make sure the other players can have as much fun as they should have by default.
The heal-bot character has to do one thing, and can't do any more, or else the game is less fun for the other players.
It's like playing a fighter, only you can theoretically do other things.
4E D&D at least made that suck less.