Why hasn't anyone made a GMail clone that works with other mail services (Exchange, IMAP, POP etc.)?

From what I can tell, it's a no-brainer.

• People love the GMail app, or so it seems.
• In my experience, 3rd party mail clients for other mail services are not exactly favourable.
• GMail's UI is actually fairly simple in terms of using a pure Holo look.
• Android's core account APIs would make handling the emails themselves a doss, I think. Not too sure on this one, I've never actually used those APIs, I'm assuming.

Maybe I've missed some vital point here. Maybe people don't like the GMail app, or there's something far more revered for 3rd party services on the Play store. Maybe GMail does handle 3rd party services, and I missed the memo. Just seems like something that should happen!

Discuss...

Edit: Okay, well I learnt something today. Because I'm on Sense, I've never seen the stock email app, which is basically what I described above!