why do baitcasting rods have a gap in the handle?

Those are called split-grip handles. It's not only casting rods that have split-grip handles all types of rods make split-grip handles. They're made to reduce the overall weight of the rod which is better because lighter rods result in less fatigue, and they also expose more of the rod blank in your hand making it more sensitive to subtle strikes.

I personally like Split-Grip handles I think they look cool. A lot of people prefer full grip handles, it's a personal preference..same thing with cork and EVA foam handles a personal preference.

Most rods today have a split grip handle. I don't like them personally.

Style and comfort. My spinning rod has the same thing.