I think its been like this for a while, I asked my friends that have been playing for years and they always tell me that this has been the pattern for years
I think its impossible for Pokemon to actually implement that system though, mostly because 80% of the set are Pokemon, Digimon works differently because an Evolution line can be from 4 to 6-7 cards for a single line, so its easier to fit 10 lines (and sometimes a certain line can fit in like 3-4 archetypes) in a set and fit the rest of the stuff needed in a set, Pokemon relies mostly on Items and Supporters in every deck and most of them are generic pieces so its extremely hard to make all playables in a set, so they rely on just doing "trash" lines that are just nostalgia bait most of the time to sell the set for collectionist or casuals
Its annoying for sure but I feel like thats a reason why this game is so cheap, Digimon meta decks can go from 100 up to 500$ for base rarity easily depending on the deck, while most pokemon decks are mostly generic low rarity pieces and like 3-4 lines max that are also very cheap at base rarity
It's been like this since its inception really. I think at first it was more of an accident, they were still trying to figure it out back then. But as the years went by, they did make an effort in some sets vs others. I've been looking at Stormfront recently and looking at rebuilding some decks out of there, because that set IMO was the best set for players. Like over half of the rare cards were playable, there are easily 12 different decks in that one set. But then you move 3 sets later to supreme victors, and that entire set is just bulk. SV had less than 10 playable cards, and it was a 150 card set! Stormfront was less than 100 cards, and almost every rare was playable in DP-SF, granted their playability went down over time, but some others actually rose later in the meta (see Gengar and Gyarados).
In terms of game design, I think Pokemon suffers a bit from it's lack of ways to play with its cards. The Standard format is great and all, and I personally like the rotation aspect as it keeps the game from becoming too stale, and makes onboarding new players super easy. But I've always been disappointed in the lack of a real sealed format. I just dropped Flesh and Blood, I'd been playing for 2 years or so, very difficult game, super rewarding to play well. But one thing that I liked in particular is their draft format. Each set has 3 "archetypes" in it (think warrior, thief, wizard and the like), so drafting felt pretty intuitive. If you are picking the warrior type, then you take the warrior cards and the generic cards that work with them. It also made the "open your packs, build a deck, and play" format pretty easy to engage with as well.
Pokemon doesn't have anything like that. The Build and Battle kit is really not a good product. You get 1 of 4 "archetypes" and 3 or 4 packs to add to it? That's not a good experience if you open a really cool ex and you have no way to use it because your "archetype" doesn't fit it. There is also the lack of a multiplayer format as well. Most TCGs now all kind of force one into their design, and I'm a little surprised Pokémon hasn't done that yet. I heard a bit about a raid battle format, but it doesn't seem like its supported well. I actually like the idea of a 4 player co-op game vs a raid Pokémon, it would be unique and also feel very similar to the video games multiplayer of late. Like if they just made it a box set or something and it just told you how to play it, and let you play with cards from your collection, I think that would be cool.
The idea here is that there is a lack of ways to use cards, so cards can only be designed in one direction. In Flesh and Blood, there are are cards you really DO NOT want to see in your sealed pool, because they need to be built around. There are rare cards that are garbage in constructed, but are some of the strongest cards in draft where passing one almost feels like a mistake. Then there are other cards that have that obligatory "target player" text, signifying that they are more useful in a multiplayer format.
All of that to say, if Pokémon cards could be played with in more than one way, sets would still have plenty of bulk in them. But at least the bulk would have a use case, even if that use case was for one set. Imo that is way less feels bad because then each card could have an opportunity shine.