well it exist because you cant just grow a digimon to 9999 and remove the challenge. (i think, i am not the game creator if you want a answer of a why you better ask him on priv)
i dont know how much digimon below mega can go
but i am sure that all of them cant get better stats than a meg
to be fair, you need to use the items to beat the challenges, if you could just train to 9999 then there is no point for the existence of boost items, aka "git gud" (sorry lol)
and as i said, digimon below mega level arent special, it just makes sense that a agumon or gatomon isnt stronger than a machinedramon in max stat
but to be fair the blitzgreymon and cresgarurumon evo is trash and needs to be lower, it was painful getting omegamon alter S
edit: i dont think you understood on the below mega, they arent special, just weaker max stat wise, they can still beat a 4 star challenge and below if you train them