How would you like it if next season your favorite champ wasn't in the game anymore? Let's pretend for a minute that Riot has decided it's too hard to balance the game with so many different champs, so instead of keeping all of them in the game half of them are removed. This is of course completely crazy and will (probably) never happen, but you never know. Riot's done some pretty weird sh*t in the past.
Anyways, this is the reality for pokemon right now. More than half the pokemon from past games won't be in pokemon sword/shield, and there is no way to obtain them. And I'm not just talking about being unable to catch them in the wild. They're literally not even in the national dex. And pokemon sword/shield won't be the only games where they do this. Gamefreak has made it very clear that they have no intention of having every pokemon in future pokemon games, even though the whole point of the games is to "catch them all". And missing pokemon isn't the only problem with this game. I won't go into detail, but I will post a link below to a reddit thread that summarizes the games numerous issues very well. The TLDR is this is just a very poor quality pokemon game overall and is just pure laziness on Gamefreak's part. As always, money speaks louder than words though. It doesn't matter how much we complain. If the game makes money they don't give a sh*t what we have to say about this.