for a second, let's assume there is a working hydraulic drive system on that bike. It must consist of a pump somewhere in the vicinity of the bottom bracket. a fluid reservoir somewhere above that. A drive mechanism similar to the pump. Hoses connecting the reservoir to the pump, the pump to the drive, and the drive back to the reservoir. Then add maybe 8oz+/- of hydraulic fluid. I'm assuming that the pump is integrated with the bottom bracket spindle, and the drive is in the hub.

Anybody have a rough guess how much that rig would weigh compared to a chain, cog, and sprocket? And you better hope that stuff is dead reliable because most of it it is housed inside an aluminum frame with no apparent service openings, not even a cap for adding/checking fluid.