That's exactly what insurance is for. Why else would anyone buy it if the postal service automatically reimburses you for losing or damaging your package without having to buy insurance? UPS has insurance included with Ground shipping, but of course the rates are higher than Media Mail.
Yeah, you're right, I shouldn't complain about the loss of my property and lack of fulfillment of a service I paid for. I should have to pay extra if I really wanted that service. Next time I go to a restaurant, I'll be sure to buy insurance to make sure the server actually brings food to me.