As an IT person I can provide you one of many possible and likely reasons.

These systems are very complex and often use command line interfaces and require special training for the user. Mapping "normal" single destination and return to origin airport trips to the required backend computer system interface is often very complex. Mapping complex flight scenarios is infinitely more complex and often doesn't justify the development cost vs. the "relatively" small number of customers that can work with a reservations agent.

HTH.