Insurance exists to provide a financial surety in the event of damages caused by some action or event.  A car hitting anything causes measurable damage, sometimes catastrophic damage, and insurance makes sense.  A bicycle hitting something generally doesn't cause much more than a couple of scratches to the hittee, notwithstanding the one or two collisions that happen every year when some ID10T hits a ped while racing on a multi-use path or something similar.  How could it - most cyclists don't go much faster than 20kph and weigh something like 90 kg, compared to a car travelling at 40kph and weighing 2000 kg, which translates to about 400 times the force when a car hits something (hey science boys, did i get that calc right??).  The potential downside doesn't justify insurance.  

And fair disclosure:  I've been driving for 41 years and have had 5 collisions, I've been riding for 53 years and have crashed as an adult, hitting a ped once.  He was charged and I'm suing him for damages.