If a driver/rider starts at the age of 16 with an affordable insurance rate, and they continue to become accident free for 8 years, the insurance company just made 8 years worth of insurance cost from them.
If they rear end someone on their 4th year, their insurance hikes and the insurance company will most likely make back all the money they spent on the claim.
I dont think the main reason insurance companies charge so much is because they are young and of a higher risk. They want to make money and try to do so by squeezing the most amount of money from young drivers while they can and while also having a valid reason that they wont get under fire.
e.g. Older operators are probably higher risk than young operators and yet they have great rates, the insurance company cannot just increase the cost because they feel that they are a higher risk. Simply because you can argue that (I've been accident free for x amount of years). While young operators have nothing backing them up. Its just a scheme.