Because that tells everyone that you need to schedule the weakest crap-fest of a schedule that you possibly can because you won't be penalized for it come BCS time. Why REWARD a cowardly schedule?
But this is all beside the point. With the system as we have it, a team should not be penalized for a bad schedule or rewarded for a good one per se. Pollsters should honestly evaluate each team and rank a team ahead of another if they think they are better, regarless of record. But that isn't what happens.
How many here doubt that there will be 3-loss teams in some conferences that would SMOKE an unbeaten Boise State like a cheap cigar?
That is where a decent schedule comes in. If you play at least a couple of decent teams, people have a reasonable standard against which to compare you. If you refuse to play anyone good, then don't whine when people disagree with your assessment of the team. If you gave them a decent standard for comparison (a decent schedule), then you'd be able to make a legitimate argument. If you DON't give them a decent standard of comparison, treat yourself to a nice long drink of ShutTheHellUp.