Mostly because there is no speed enforcement or even the threat of speed enforcement for bikes on paths, trails and bike lanes. And nobody wants to pay for that either. If there wasn't the fear of fines and points for speeding in a car, the roadways would be more dangerous than they are.
It would be great if speed limits worked for bikes and we could do away with classes and all that mess, but the limits we have now don't work. It costs a municipality a couple hundred dollars to install 1 speed limit sign. Multi use paths often post speed limits at trailheads on information boards. The most common speed limit is 15 mph, I've seen limits as low as 9 mph on urban paths. People don't see it or just disregard it. I don't think anyone would be happy with 9 to 15 mph limits on all cycling infrastructure. Those are the limits we would get. Think of the automobile limit of 15 mph in school zones. That's what you get when there are a lot of kids, pedestrians and pets. We have big county parks with roadways and a posted speed limit of 15 mph. That's what we would get on cycling infrastructure and I think an enforced limit of 15 mph would anger and frustrate more people than the classes do now.
Just my opinion, but I'd rather spend money on more miles of paths and trails than speed enforcement.