Character backstory - Better Source DM or player?

[KorvinStarmast]

The question is, if people show up with characters that don't have a backstory, do they even need an incentive to stay together? In my experience that kind of players leans toward "we'll adventure together because we're the players."

For a lot of people, that's enough. For some people, it isn't because they are looking for a deeper level of immersion into the character, and may assume that all of the other players do as well, which is only rarely true. (IME). The desired level of immersion into the character varies by player.