Because other characters can attack with them.
I think that would be pretty reasonable, but in that case I would have it only affect one stone at a time that only you can attack with.
I had thought this may be the reason, but you don't create more as you tier-up either
given it's a bonus action they probably just expected characters to be creating a turn-by-turn supply of them.
Its supposed to be ranged shillelagh, which also doesn't scale with level but with # of attacks. In theory, if you can increase the number of attacks you get, it matches the cantrip progression. But unless you are a high-level fighter with Magic Stone as a cantrip, it rarely works out like that.
i know fighters very rarely actually get to EA+3 but magic stone doesn't even create four stones, only three, which is even more of a let down from the spell.
Three is already enough for an 11th+ level fighter to use for all of their attacks.
True in the 2014 rules, but in the revised rules, Shillelagh also increases in damage die at cantrip scaling levels - it’s a d8 at 1st level, d10 at 5th level, d12 at 11th level, and 2d6 at 17th level. So, there’s a case to be made that magic stone should have scaled similarly, going up to d8s at 5th, d10s at 11th, and d12s at 17th.
It doesn't scale because it can benefit from things like Extra Attack, and the typical cantrip scaling is thought to be too much if you can do it twice or more a round.
