One would think so! Yes, it seems strange to me too, but in the past four months of watching eBay, that's just the way things are. Take Mark IV's, for example. Now there *have* been exceptions lately, but generally heads sell for $1400-1600 while combos sell from $1000-1400.
My guess is that the majority of people pursuing these amps either A) already own a cab, therefore won't want a combo, or B) simply find a head more desirable, due to versatility (as in being able to try matching it with various cabs) or even something as pointless as thinking that a head/cab looks "cooler" than a combo.
I found myself in the second category, but decided to buy a combo nonetheless. I found that, given this would be my first Mesa and even first tube amp, I was unable to justify spending $1400+ for a head and $350+ for a cab when I could buy a combo for $1400 or less...
...well I still want a head/cab :x
I think what I'll do is continue to explore my Mark IV combo for the remainder of the summer, then sell it at the end. This allows me to switch to head/cab as soon as I get back to college. Sure, there's a bit of a wait involved where I'll be amp-less, but I don't mind