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Not sure it's 90%, but many are in my area too. There is a bike path on which I often walk and occasionally ride. A majority of the spandex-clad twerps who weigh in at about a buck fifty, with bikes that cost about $2k, are completely discourteous.
The bike path is heavily used by people jogging, walking their dogs, pushing strollers. Most of the "serious" bikers cut between two groups of people walking opposite directions, with no warning. They can't wait five seconds for one group to pass--they have to keep up their "pace."
When I ride, I use a bike bell to advise others when I am passing. Simple courtesy. Apparently too much to ask from most bikers. :(
I avoid riding on the streets. Too dangerous. I've had enough road rash over the years to prove it. I stop at stop signs out of self-preservation.
Which is why serious cyclist avoid the bike paths. The one time my wife was injured riding her bike was on a bike path. She was going dead slow, but a little kid on his bike erratically shot across her path and so avoid him she crashed her bike, cracking her elbow. The kid's father didn't even acknowledge her. On the street, at least everyone's going the same direction.