because they don't want to get used to playing with addons because it might effect their tourney play where they can't use addons.
this number of these players is so infinitesimal that they are not a very good sample to go by. a tourney capable player is miles above even most multiglad players.
for the vast majority of us, from random bg heros to players consistently capable of 2.4k addons are a significant quality of life boost and can often improve the quality of our play.