Why did they put the Chipettes in the second film?
They could've easily of had a spin-off after the first Alvin film and then mix their "spin-off" with the second Alvin film, which sets the events on how they knew about the Chipmunks and Ian.
Or after the sqeakquel (which would've been a different plot), then make the spin off.. OR EVEN after a trilogy of ONLY CHIPMUNKS films, have a spin-off for the Chipettes and put them in the fourth film which could've been the plot of Squeakquel, the fifth film would've had the events of Chipwrecked.
They could've made so much more money that way! What a wasted opportunity.
I would claim two reasons: 1) a film with just The Chipettes would probably not be as successful as one with The Chipmunks or a combination of the groups and 2) I doubt there were ever plans for four, let alone five, films. I don't see a problem with how soon or in what manner the girls meet the boys. The groups met soon, the second episode, in the 80s though they didn't know about each other at their meeting. Regardless, more than a billion dollars is quite a paycheck for the companies involved.
Also, the demand for the Chipettes to make a return was always pretty strong. One of their behind the scenes thingyo's for the second movie said they knew, if the first film was a sucess, they would include the Chipettes in the second.
I know what you mean, though. It also would have been cool if they'd released little 22-minute films (like they did for How to Train Your Dragon and The Smurfs, etc) to accent the film... but it's such hard work making the Live-Action/CGI an animation, I don't blame them :/