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.

Transcript

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.
Anakin: Um—
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.