https://en.m.wikipedia.org/wiki/Family_History_Library

Googling this sort of thing is fun: the first result I got was this quora page (it’s fun to read the nonsense sometimes, sue me) where all the responses are very salty and defensive mormons to what reads to me as a fairly worded question.

https://www.quora.com/Why-do-Mormons-have-the-biggest-DNA-database

It seems to boil down to they believe you can save your ancestors from damnation if you baptize them post mortem. Regardless of how far back.