futures: Probably because:
1) Companies can make loads of profit selling their proprietary batteries -and-
2) because it's a heck of a lot easier to make something that's the size and shape they want, when they control the size and shape of all the components including the battery. This is especially true with handhelds and phones where a couple mm is the difference between something being "thin" or "thick"

/me dreams of little radio-isotope batteries that give 20 years of runtime

-John