Why are bicyclists required to ride their bikes on the road?

I know that it is hazardous for the pedestrians if the cyclists drive on the sidewalk, but wouldn't riding the bike on the road be hazardous to the cyclists? I mean, say that you're on a 25-40 MPH road, most likely you would not be able to ride at that pace(25-40 MPH). This would lead to severe traffic for the motorists and create on-road perils. I always witness cyclists getting ranted at by motorists by getting honked at or told to "get the f. uc.k on the road!". I heard that people ages 12+ are required to ride their bikes on the road. WTH? It is very unsafe for children ages 13-15 to ride on the road, they are just kids. Making turn signals with your hands and having to watch your back constantly would also be such a hassle. Last but not least, accidents from cyclists riding their bikes on the sidewalk is not that common, at least I haven't heard of one accident involving a cyclist riding on the sidewalk. BTW, I live in NJ so the laws in your country may differ.