I don't understand why the point you're raising makes it so that starting players at 0 MMR is a bad idea?
In the short term it would have the effect you want.
In the long term it would not. If every new player started at 0 over time 0 would become the new mid-point.
The tradeoff being made here is mostly that this is the quickest way to place people accurately. You can do things to make it less accurate (artificially set players MMR, force restrictions on who they can play against) but these make the system take longer to find their true MMR, and this extra time further compounds into less accurate matchmaking in the long term. We make some of these compromises (there are soft restrictions in place to prevent new new players from facing veterans) but Matchmaking systems are quite delicate as they rely on assumptions and breaking those assumptions can lead to unexpected behavior and wonkiness in the long run.