Because splitting up into just two separate groups isn't enough.
Some GMs are, through hard-won experience, prepared for the party splitting into two groups and can handle it with aplomb and dignity. This is when you need to pull out all the stops.
Qui-Gon: We should split up.
GM: Oh god...
Qui-Gon: We'll storm the palace and get Goatee Guy. Jar Jar, you take—
Obi-Wan: Jar Jar's holding off the droid army on the battlefield.
Qui-Gon: Right. Annie, you take that fighter and infiltrate the orbital samurai hideout.
Qui-Gon: Your job is to assassinate the space samurai boss.
Obi-Wan: Oh, sure. Why don't you get her to single-handedly destroy the base, and deactivate the Lost Orb and bring it back while she's at it?
Qui-Gon: Ooh, yeah, good idea. Do that too.
Anakin: But I'm just a boy!
R2-D2: Don't worry, kid. I'll drive.