I don't know of any off the top of my head due to one issue- cost. The most expensive item on a mower is the drive motor and no manufacturer wants to build a unit that won't be price competitive. Also, most motors won't run well at the higher RPM's required by the blade spindles for good cutting performance I don't think. The over-driving characteristics of a belt-driven system increases your blade tip-speed simply through running a big drive pulley and small driven pulleys.
I believe that weight and complexity would be an issue as well.
UFM82