You can have a primary deity, and that more or less (in a normal person in toril sense) represents either your profession or region or race or alignment...often a mixture of them all. It doesnt mean you absolutely refuse to worship others, and in many cases praying fervently to those deities you need...or necessarily hate those who sit on the opposing spectrum. In these cases their primary deities are more what best suits them to living good in that god's eyes without drastically changing their lifestyle they were born in. Becoming "faithed" by it's definition in the game, is a whole new level of dedication.
By Ao's edicts, deities have a responsibility to respond to their worshippers...worshippers doesn't equate to "faithed" as in the game mechanic. All it requires is realizing you're at their mercy, that you need them to truly succeed, general shows of humility and sacrifice to a god. Elves, Dwarves, Gnomish, Orcish, any race's pantheon that isnt the primary pantheon, worship all of their gods...moreso than the primary pantheon, but there is still many god worship among humans and the primary pantheon...just not as equal or obvious.
I'm not exactly sure why they structured it that way in FR, I have a gut feeling it's a stereotype of our abrahamic monotheistic dominance in our world...having the other races worshipping more equally among their pantheon I think was to distinguish them apart from Humans culturally.
The unfaithed is a pride issue. It isn't that they don't believe they exist, or that they feel none will help them - it's that they refuse to admit they need help, or truly believe they are good enough without divine will. Karsus' folly was a pride issue. The greatest virtue, and greatest sin, are the same for all the gods of every pantheon - obedience/pride. Which means when talking about the unfaithed in Kelemvor's judgement, is different than the agnostic rating that is a mechanic in the game. You can be humble to many gods, all at once, equally - that is being in faith....you're probably best staying in one pantheon though.
