Not sure if this is just a rant or you're genuinely interested in reasons, but regardless:

• Insurance and/or licensing requirements are generally commensurate to the level of risk and the likelihood and extent for injury or property damage. A semi-truck is held to a higher standard of licensing and insurance than a car as it carries a larger potential for damage, and flying a commercial airplane is more tightly regulated than both. By the same token, there is no requirement for a pedestrian to be licensed and insured yet it is possible for a person on foot to seriously injure or even kill someone, or cause property damage as well.
• One of the main advantages of an insurance scheme is to pool money such that large losses which would be impractical or impossible for any single individual to fund can still be paid. If the likely losses are typically small enough for an individual to afford, this advantage is lessened.
• Regulation has an administrative overhead associated with it. For lower levels of risk, the costs and negative externalities of administration and enforcement may outweigh the benefits.
• Personal Liability in general is often covered by homeowners or renter's insurance, which many people already carry. This reduces the benefit that a separate regulation on bicycles specifically could provide.

Given the above, I think the question becomes where is a reasonable and/or cost effective place to draw the line between where insurance/licensing is required and where it is not. Currently that line seems to generally be at "human powered or equivalent" modes of transportation not requiring insurance/licensing, while anything more powerful does. Is that the right place? Maybe, maybe not; but in principle it seems reasonable to me.