They are not sequels to the Alien films: https://www.themoviedb.org/bible/collection

They're prequels. Are prequels excluded?

The Star Wars prequels are in the same collection as the original trilogy.

They will be linked nicely with proper Universes at some point in the future. The Star Wars Collection is an exception. (But imo, they should also be split)

But would you put the new sequel trilogy (7,8 & 9) in the same collection as the original trilogy while splitting in the prequels into their own collection? That seems messy.

I don't understand why prequels aren't treated as sequels, since technically a prequel is a type of sequel ("prequel" obviously being a portmanteau of "pre" + "sequel"). And there's no distinction made in the bible there...

 But would you put the new sequel trilogy (7,8 & 9) in the same collection as the original trilogy while splitting in the prequels into their own collection?

Star wars? No. I would split them into three collections. The new trilogy might be set after the original trilogy, but they focus on a different/new set of characters.

One of the issue with prequel is they are often a new production with different actors and whatnot, and/or the start of a new series of films.