to play the 5th and 6th best teams instead of teams 3 & 4? What if teams 5 and/or 6 are from those same conferences? Would you want the top two teams to play teams 7 and 8 in the playoffs?
The whole point is to put the best 4 teams in the playoffs and if we don't recognize that it is possible-even probable-that more than one top 4 team may be from the same conference then you are diluting the playoff.