Because your ranking should be a true representation of your strength as a team. If you play a strong schedule and have a great record, your record is a good indicator of how stong of a team you are. If you play a weak schedule, your record means a whole let less when evaluating how strong the team is. The number one team should be the best team in the country, not a team that makes everyone feel good beacuse they are a cinderella story with a strong record against weak competition.
Eh. It never is. UConn was a 2-seed going into the 2004 NCAA tournament, but everyone thought they were the best team. Even in 2011, while UConn was a 3-seed, a greater percentage of people were picking them over all but a few teams. Subjective rankings never have been, and never will be accurate. Not everyone plays everyone else, so we never
really know how strong a team is. Miami beat Michigan State, and Duke (without Kelly). They've also lost to Florida Gulf Coast (146 in KenPom), Wake Forest (123), and Indiana St (95).
And, for all the talk about SOS, remember that the RPI doesn't take into account margin of victory. KenPom or Sagarin are clearly better indicators of a team's strength than RPI. They are unbiased, and KenPom has Gonzaga as #4 and Sagarin as #6.