Main reasons as I see it:
-Compromised performance
-Small screen (relative to HDTV) so less suited to split screen
-Single kb and mouse, typically don't have multiple controllers (compared to consoles where the majority of owners will have multiple controllers)
-Typical setup isn't as conducive to split-screen action i.e. computer desk with single computer chair maybe in a cramped office compared to huge living rooms with big sofas etc
-Historically not that common so publishers probably don't see it as a key feature they want developers to put in
-PCs lead the online gaming revolution - most games of that type from the past ~15 years have supported online multiplayer whereas it wasn't really until the XB360/PS3/Wii generation of consoles about ~7-8 years ago that consoles started to fully embrace it. The implication of this is why bother catering for splitscreen when proper multiplayer is going to be much more useful