you may not, but the people who design the sets take special care, and as far as i know almost always have taken special care to ensure that every pokemon card can be played using cards from the set you pull them from.
moreover, there's a bit of a misunderstanding of the point. the point is that every generation after 5 (and 5 after they buckled in bw2) has a significant, if not overbearing presence of previous generation pokemon practically crowding out anything actually new they came up with. the galar pokedex for example has 400 pokemon in it. if you took out everything that wasn't a generation 8 pokemon, you would be left with a pile that you couldn't make a set out of that actually represents sword and shield as a game well at all.