They used to be replaceable for a long time, I think the main advantage of having them inside the phone and not replaceable is that it needs less space, normally the battery comparement it shielded of by plastic for instance, this is most likely already a mm think.
So it's just a typical case of apple using form over function...
No, it isn't.
A phone with a replaceable battery is significantly thicker, heavier, and more mechanically complicated. Thus, most people will find it not only less desirable; they'll also find that it's more prone to mechanical failure.
All that just to please a very small percentage of users who want more flexibility.
How hard would it really be to have a sliding back cover like the iPhone 4 had that gave you easy access to the battery?
No iPhone had access to the battery. No iPhone had a sliding back cover.
I'd gladly take a phone that's 2mm thicker if I could replace the battery freely... thin phones just feel slippery in the hand...
It's more than just thickness.