My question is, after the twitter fiasco, there is a number of competitors, like skyblue or this threads that have been created to remplace it (I dont count Mastodon, because it was not created to remplaced, but as an open alternative).

Why absolutely no big company (MS, facebook, google, amazon, etc...) is attempting the same with Reddit? Not even discord has done any single change to try to steal any userbase from it.