Why is 大家们 (dàjiāmén) a thing?
大家们 is not correct.

Not all search terms with a lot of results are correct words/phrases.

You should notice that there are not many meaningful results in the search results. And the very first result is a Q/A site asking whether "大家们" is correct, and the accept answer says it is not. The second result is a paper from a journal that discusses this kind of errors.