This is kind of a really funny way of putting this. "You shouldn't have to buy a game. You should just be able to buy a game!"But no one should have to buy an entire game/dlc for a non-mythical mon. Or just do as Yung Dramps suggested and release ways for us to catch and transfer the mons we want.
Like, that is the situation we are in here! If you want the 6 Regi, the wolves, Eternatus, Calyrex, the Galarian Birds, technicallycosmog&cosmoem, the forces of nature, all the other DP legends...strictly speaking if you want Mewtwo & Dialga/Palkia but missed their raids (I'm a little more lenient on these 3), too. You gotta buy SWSH + the DLC* &/or LA depending on what you want (BDSP also an alternative option for the older legends).
To me the thing that makes it annoying isn't the need to get another game it's that those Pokemon are "in the game" and there is a provided method to obtain the assorted Legendary pokemon, but only some of them when traditionally it's all of them*. It's like the moment they had cross-generational transfers that wiped movesets they immediately stoped caring and just picked an assortment of Pokemon. If they had the moveset wipe implemented to begin with, I can easily envision them doing the same thing in Gens 6-8 (8 did sort of implement it but much later on).
Ideally they'd do more raids but after all this time I doubt that happens.
*In some manner of fairness, while Calyrex is probably "the big one" you'd buy it for, SWSH+ The DLC does give you access to literally everyone but Enamorus (& an extra Kubfu).
**Although to be clear I don't particularly care that they decided not to literally have EVERY Legend back. Cutting the UBs, Tapu & Kalos box legends is...fine. A little odd, but whatever. It's not like they're there.