Probably because very few folk own a K7SEM. Like stated above, you're trying to compare apples to oranges here.
The K7SEM is an integrated "all-in-one" solution with onboard video, sound, and LAN (IIRC) and using low-end SDRAM. Targeted more for the OEM market and cheap "grey-box" systems sold at your local store.
The K7S5A is the non-integrated solution for the custom builder, without the onboard video and the option of using DDR RAM. While it doesn't overclock as handily as some of the other solutions out there, it's still a pretty stable board with good features.
Now, that said - why not grab a KT333-based solution (or KT266A if price is an issue)? Those boards overclock like champs and perform at or above the level of the SiS-based K7S series.
- M4H