I think at some point earlier last decade, Theo Epstein told the BoSox fans that they would have to possibly rebuild and may not be contending for a bit. They did have a couple decent drafts pan out though. It's not unheard of.
The difference is that the Yankees and Red Sox are very very smart and also care solely about winning (for the Red Sox it's more recent obviously
). They also spend quite a bit on drafting, scouting, player development and other stuff besides just their major league payroll. The Yankees core is getting older but was basically Bernie Williams, Derek Jeter, Jorge Posada, Andy Pettitte and Mariano Rivera for a very long time. The Red Sox have a bunch of home grown guys in their everyday lineup. Neither team has had to rebuild all that significantly because they invested such that they could have that core to build off of.
With the Cubs, they buy up other teams' free agents when they get expensive while their former teams continue to find other ways to win. There doesn't seem to be very much in the way of prospect development. Whenever they get a crop of good ones, it seems like they decide to just trade them for someone else's homegrown, about-to-get-really-expensive star. That reduces the window of contention to those years when the free agents signed are still in their primes, and then when they break down, they're left with subpar players and expensive contracts that nobody wants.
So...obviously that's not working. Why not rebuild?
By the way, someone might want to look up that Theo Epstein situation I was talking about, I'm not sure I'm totally right, but I do recall him saying something like that.