probably because no one uses them. You can get a 4GB thumb drive for like $8 now, why burn a bunch of stuff to one disc once?
But by the same token, you're spending $2/GB on a really tiny medium. My Mac Pro has 6TB spread across the four bays and I don't think any of the drives were $2/GB. Plus, like I say, a 4GB drive only holds 4GB. You can spend a lot of time encoding and compressing... or you can just throw something in a directory where you can find it and the other few thousand items you've got stored.

(And as I start to see the day coming when I'm going to have to start managing space, I'm starting to think a BR drive from OWC would give me a storage option for lots of stuff I'd like to have but is in the need-to-be-in-the-mood-for-it category... I don't really need to keep it on a media server as long as I can find it when I want it. That would be a metric buttload of really inconvenient thumb drives!)