I'm guessing is mostly because of the amount of wasted stats on a really expensive item. Sure, the sheen proc is neat, but you're not using the AP nor mana at all and riven doesn't scale well with attsp nor movement speed (from the phage passive), as she mostly deals her damage and moves via her abilities. She also prefers AD over health, since she got a ******** shield scaling from AD.
You also forgot that she isn't reliant on the 10% critical either. YGB has 15% and 20 penetration along with 10% CDR. Plus the active is nice for skirmishes. And no stats are wasted gold and it's about 1k gold cheaper than TF.
Other than that, yeah what he said.