Some have those accounts indeed. Yet
- They are way, way, less many Bronze to Masters streamers than freelosing smurfs. To give you an idea, over the 183 smurfs I screened in 2020, there was only one alleged streamer. And the real proportion is probably much lower.
- Those tend to freelose all in one go (amount of which could be lessened if using a new account). Freelosing smurfs do this repeatedly. So that means Bronze to Masters series would become more tedious to set-up, but impossible, and at the very least not as punishing as it would be to regular smurfs. And this supposing the Bronze to Masters series are not already long done.
- if you have a GM or more execution, you will win often against players several leagues under your level, even using silly strategies (Crank for example won against a platinum player using a mass probes). And so the legitimacy of doing Bronze to Masters series to prove the efficiency of some builds is more than questionable.
- If pros could use smurfs accounts the way they are intended to, and regular players could have the matchmaking work the way it is intended to, this would be greatly beneficial for the enjoyment of play of regular players.
Overall the game is mainly aimed at players, so even if not that relevant Bronze to Masters series had to be sacrificed in order to get a more enjoyable game for most, then it’d still be a quite beneficial trade overall. 
Anyway, I already said the devs won’t do a thing against smurfing. Why are we even arguing about possibilities that won’t be ? Fret not, toxicity is here to stay. 