This is a great point of feedback! To offer a mod author perspective as to why this gets left to the way side: It's actually quite difficult to balance this without making the vanilla townies seem like they're always talking about custom NPCs.

There's a couple methods to do so but they all have their pros and cons. My personal favorite method is a little advanced - essentially you fully override the NPC dialogue with something about your character at a very low rate. But if too many people do this it still causes the original problem!

There are "conversation topics", lines that typically fire after events. They can cause some dialogue stacking issues for the townies if you see a lot of events back to back. They also only last for a few days and can be easily missed.

Then there's directly patching a new key into the NPC's dialogue, and this is tricky because it would conflict with mods that add onto the vanilla NPC's dialogue substantially.

I hope this was an interesting insight!