Because if building a team around Jason Kidd or Gary Payton didn't yield a championship, it definitely won't happen with a team whose best player is Rondo.
You mention having "a couple great scorers," but a truly 'great scorer' is likely to be better than Rondo, and thus the centerpiece of the team.
As others have said, Rondo is very good as a #2 or #3. Not a player you build around.