why route must always depart at your base airport ?
The manual explains the air traffic rights and how they are modelled: https://www.airwaysim.com/game/Manual/Routes/Freedoms/
And base airports can be used to expand your business: https://www.airwaysim.com/game/Manual/Office/Bases/
Suppose another question is why cant we do W patterns. For eg, here in ireland we have a turboprop ATR72-600 that flys ORK-BHX, BHX-SNN, SNN-BHX, BHX-ORK.
BHX or SNN is not a base airport.