Because OST is the most babied faction on the game. They've been given so many advantages over the years that they were never intended to have.
Right now the best stock infantry units belong to OST. The LMG42 Grens with the early Panzergrens is more effective than BAR Riflemen, Bren Tommies, or Penals/7-man cons. It even beats out the Volks/Strums/Ober combo due to timing on Obers (and the lack of dual schreks).
This is hilarious when combined with the best MG in the game, T0 only 260mp. Mortar, and sniper too.
You make it sound like grenadiers are terminators or something. I know their LMG makes them scale pretty well in terms of damage but come on, they are a 4 man squad and even with vet they end up being way less durable than most allied infantry.