People with healthy diets don’t need them to, and there are advantages to teeth secured very tightly into our jaw. Teeth are actually very strong and resilient.

And when people do start losing teeth, it mostly happens past the age of reproduction to the evolutionary impact is lessened. On an evolutionary scale, we’ve only fucked up our diets with sugar and processed foods very recently. Plus, we now have dentistry to reduce that impact, so I doubt evolution will make it happen to humanity in the future.

There are actually people that grow more teeth, but they have more complications than advantages. I suspect this has been the case historically as well. If everyone else can do with just the normal amount of teeth, these people don’t get an evolutionary advantage and their teeth gene quirks don’t become common among humans.

That said, scientists are working on stimulating teeth growth for people who have lost them or never had them. It’s not impossible, it seems, just very difficult.