Probably because we aren't willing to pay airplane prices and do maintenance on airplane schedules. I suspect that a container ship usually takes longer to travel between two ports of call than an airplane is legally allowed to go between inspections.

They happen regularly! That's why you have checks between flights, no mater how short the flight. Daily, weekly, monthly and by cycles(flights) or hours maintenance. You have leaks, breakage, electronics get fried, conexions get loose. Aircraft parts are numbered and tracked they hace a limited live span depending the function. I can not imagine an airplane wirking properly and safely without all this care.

Airplanes have pretty low reliability - multiple maintenance man hours per flight hour. But their time in-transit, equivalent to time between ports for a ship, is measured in hours, not weeks.