If I had a single-car crash with minor injuries, why would I report it? It seems like there’s limited upside to me as the (obviously at-fault) driver. Go get medical care and go get my car fixed. (I drive cars cheap enough to replace and so do not insure them against collisions that are my fault.)

Drivers driving without a valid license, without insurance, under the influence, wanted by the law, etc would have all kinds of reasons to not report a crash.