Because there are way too many possibilities for it to be worthwhile. Mathematically speaking, you have 2 to the power
of n possible outcomes left in the tournament (where n is the number of teams left in the tournament minus one).
So, if there are 32 teams left there would be 2.1 Billion outcomes which is why we wait until there are 16 teams or
less (32,768 possible outcomes. A lot, but not unreasonable).