I'm not sure if this answer provides a root explanation for the question, but in a competitive (but not prestigious) undergrad business program the answer provides is https://en.m.wikipedia.org/wiki/Time_value_of_money
Interest paid is offered as compensation for the opportunity cost the lender experiences by lending rather than immediately spending. I still don't see why this implies compound interest, except that vaguely the opportunity cost increases faster than the passage of time.
Bald eagles are cool but have you ever heard what they sound like? Shame that they're claimed by burger land as official bird tho.