It's a recurrent meme. Not that I like to play like that, but I've seen it many times. Some DMs that insist you fill up that space, even if playing on a homebrew setting and some people I've played with consider it a necessary thing to have a complete character. And like [MENTION=6775031]Saelorn[/MENTION] said, many character sheets, fanmade and official feature Deity on a very visible place.
Recently [MENTION=6778044]Ilbranteloth[/MENTION] mentioned in the FR thread that "everybody picks a patron" is intrinsic to D&D and not a Realms thing, and it got me wondering about where did it come from.
It's a very common assumption, and has been over multiple editions. I just don't think it's ever been required.
The gods are known to exist as a matter of fact, not a matter of faith. They are known to reward their faithful in the here and now, and presumably reward you in the afterlife with a place in their domain.
Thus it is assumed that most people (NPC's especially) will choose to have a patron deity. It's the most reasonable course of action.
