Seems tougher to do estimation specifically for motorcyclists.

Some areas allow lane splitting, others don't. Some riders are on tourers that can't hop through traffic but can maintain highway speeds comfortably, others ride single-cylinder dual sport bikes that can't maintain 60 without taking a physical toll on the rider. And every rider has a slightly different risk tolerance, which translates to different behavior in traffic. I personally don't mind lane splitting between two semi trucks cruising abreast at highway speed, but I get anxious squeezing through cramped city traffic. And I slow down considerably anywhere a SMIDSY might happen, even if I have the right of way.

Also, it may be difficult for Google to identify who is riding a motorcycle from location data alone, which is what they would use to generate the motorcycle estimations. Not sure how they would do it...maybe using accelerometer/gyro data to identify inward leans during turns? The assumption being that cars will lean outwards, while bikes will lean inwards.

They did that recently in India - not sure if and when they'll roll it out to the rest of the world.

I sometimes got the impression they do but still present it as car. I've got highly unrealistic driving times of a car in Bangkok that were very close to my actual driving time on a scooter for example.