Because it's a decently popular, yet niche archetype that people like to play and doesnt cost an arm and a leg to collect. The main reason it's always a rehash is because if you DON'T rehash it, as in, use the same attack twice, Expanded will be a hilarious race to see who can mill 24 cards of their own deck first.
You do realize that if my suggestion is followed you're left with literally zero of the problems you made up, right? It is mind bogglingly obvious that by "don't rehash it", I didn't mean "use the same attack name all the times you repeat the archetype", I meant create an entirely new archetype. "Dump mons on your discard pile for damage multipliers" cannot be the only way they can design a "decently popular [...] archetype that people like to play and doesn't cost an arm and a leg to collect".
In fact, the biggest problem is not even creating different things, we had many attempts at the "dump x things on the discard" or "discard x things from hand", etc etc "single prize cheapies" trope, the problem is that it's never good enough to be "the new night march" (some) people hope for, hence the discussion people were having about it having 20x or 30x multiplier.
And my opinion is that the 20x X 30x argument is stupid to begin with.
First because it is obviously a very "all or nothing" way of approaching the design, and it should be clear to people that have played the game for so long, that simply tweaking the numbers on an archetype like that, risks rendering the deck either useless or completely broken. There are other "knobs" you should consider adjusting to adapt the power level of a deck like that, other than just the most glaringly obvious one.
And second, because like I suggested, other than trying to have a go for the 4th bloody time at the same archetype and bothering having to figure out how to adjust its power level to current times, why aren't they designing new ways for single prize decks to be competitive again? And I don't mean the stupid "dump 20 tools from your hand" or "have 12 of these on your discard" decks, I mean actual, old school, evolving, single prize decks. Think...PTCGO's Legacy (r.i.p. and r.i.p.). That's what a good format, with good balance between multiprize and single prize decks, should look like.
Sadly, I don't think we're ever coming back to something that resembles legacy in that sense, of having that good balance between multi and single prize options. I don't think we're gonna have something like Legacy's Empoleon, or Garchomp, in standard. But I mean, we should be glad that by bringing old school exs back, we can go back to at least being able to play into one the single most important and emblematic fantasies of the whole IP again: evolving the damn things. That is, being optimistic and assuming the stupidity that is Lost Box won't gatekeep stage 1 and 2 exs for a whole year before it rotates and is no longer a blight in the game.