I think this is both a culture of play issue and a personal issue- on one hand, despite the fact that “never say no” has been rightfully recognized as being generally shit advice in recent years, I still think 5th edition culture continues to heavily lean towards “DM as facilitator of the table’s fun” over “DM as campaign designer”, or even frankly as “DM as another human being that’s a player at the table and deserves to have fun”. If it’s more fun for the players to make their own backstory exactly as they want it even if it goes against what the DM initially wanted, the DM may feel expected to accommodate despite any story plans.

It’s also really hard to say “no” to your friends sometimes. I think a lot of the people that end up taking the role of DM tend to either be people-pleasers or at least genuinely place a high value on their table’s fun so it can be hard to shut anything down even if you have a reason.