You and your players have a conflict of expectations.###
Depending on who you ask, D&D means different things to different people. For some, D&D is a combat-heavy game where the DM says what monsters appear, and the players roll dice until the monsters are dead. For some, D&D is game where the DM writes and advances the story, and the player characters come and go without consequence. For others, D&D is a RP-heavy game where the DM serves as referee, while the players shape the plot via their characters and choices.
All of these approaches are valid. So what's the problem?
You need to account for the various preconceptions that your players bring to the table. Player 1 thinks putting effort into their backstory will have payoff. Player 2 thinks they should have narrative control over a minor NPC that they created. You (the DM) think that the players shouldn't influence how you write NPCs or plotlines. The players are not wrong, but their expectations are not compatible with your own.
Communicate. Discuss your expectations with your players. Then play.
You need to establish the expectations for your D&D game. Communicate to your players about the separation between player input and DM input. Or, perhaps you could present this as an open discussion, and possibly adjust the DM role if the players want more narrative control.
Often it's practical for the DM to delegate a portion of the world-building work to the players. But it's your campaign and ultimately you decide the DM's authority over the game world. Either way, you and your players need to get on the same page.
Many groups organize a Session Zero to discuss their expectations for what they want (and do not want) in the campaign. Typically the Session Zero should occur before the campaign begins, but it is entirely possible to have this discussion during the campaign.
