Because the FCC decrees it as such.
Wait...I thought "all the radio manufacturers" programmed them the same, per A above, and just in a weird order to your tastes? I'm not saying it doesn't exist, but I've never seen a handheld GMRS or FRS radio where channel 1 wasn't 462.5625 across the board, channel 2 wasn't 462.5875 across the board, etc...
It can get weird with some of the mobile radios that choose to omit 8-14 (where they can't transmit) and so channel 15 becomes 8, but the frequencies should all line up.
Plus mobile radios, if you're into that sort of thing. And the handhelds can often be had in higher quality and with more bells and whistles than most FRS radios. But from some points of view that's a detriment.
As was stated above. FM = wide, NFM = narrow.