Because the SBMM playlist is supposed to put you into your ranking before everything else becomes SBMM.
That way there isn't such a stark contrast in all PvP modes. This is why the most popular PvP mode was selected to become SBMM.
This is also why penalties are in place. It's not that it ruins the experience of other players, it's because leaving doesn't help what Bungie is trying to do.
People are just going to stop playing before they even get there. A quitter penalty on a social playlist is dumb. We already know what the end result is going to be