Because it’s completely unnecessary.
If you watch streamers regularly, you will know that every time a new hero gets released or a hero gets reworked into something completely new, at high GM ranked games, every single game there will be masters/GM players testing out the new heroes for the first time, in a high RANKED masters/GM game.
These players don’t test new heroes for the first time in quickmatch or AI. They test it in high ranked GM games straightaway.
And these players still play at masters/GM level anyway. Those dumbos in your games, you let them test their hero to level 50 they still play like dumbos. Level is completely irrelevant.
It’s just the reality of life.