Why is there a magical barrier between pre- and post 30 games?

There's level 30+ players that are worse than the average 30- player and players below level 30 that are better than the average 30+ player. Let's call those players exceptional for the sake of this discussion, while we call the others average. Currently, the matchmaking assumes everyone is an average players, so they match the 30- against each other and the 30+ against each other. If you are an exceptional player, the matchmaking uses mmr to match you against other exceptional players, or it gives up and matches you against average players. Removing the mechanic of separate matchmaking pre- and post 30 would make it much better for the exceptional players and the average players that gets matched with them. The average player will still play with people with similar skill level, thanks to mmr.