But a lot of people out there do have fairly powerful rigs, and if the popularity of MGSO is anything to go by, a version for Oblivion would be used by a lot of folks.
I play Morrowind, I have tons of graphical mods installed including MGE XE with custom and tweaked shaders, and I don't use MGSO.
MGSO is popular because Morrowind has a very bad modding infrastructure:
1) finding mods for MW is pain, due to its age mods are scattered on a dozen of sites, some of them went down, others will be soon, and the rest is a complete disaster to search and make decisions (like PES or MMH) from user experience pov.
With Oblivion there is only 1 place to go - nexus, with good search, sorting, top lists, categories, explicit descriptions, screenshots and user comments including absolutely essential information on compatibility and troubleshooting. Try to imagine what mod you want in Oblivion, and it'll take 5 mins top to find it on nexus and know what mods it probably won't work with. Now try to do the same with Morrowind... I wish you good luck. There is nexus for Morrowind, but it's too young and has probably only 10% of mods.
Like Gabe said "piracy is a problem of service", the same principal applies to modding "mods compilation is a problem of service". Oblivion doesn't have a problem with service.
2) modding Oblivion is much easier, you can swap mods back and forth any time, and there is a very low chance that your savegame gets corrupted. In Morrowind you can't do that due to a format in which references are stored in savegame, you'll get duped objects and other nasty stuff after the first swap if you don't repair your savegame properly. Having a ready and stable compilation in this case is a great advantage. And this again is a problem of service.
The bad thing is, Skyrim took a step backwards to Morrowind with it's brilliant idea of storing the whole papyrus virtual machine stack in savegame, and now people getting CTDs and bloating after swapping mods.
I bet mods compilations will be very popular for Skyrim in future. For Oblivion? Please no.