when A->B costs more than A->B->C you got to wonder where the logic it.
time to regulate the airlines industry so that this bullshit stops.
Probably because airlines compete on endpoints and this is what determines the fare but their cost is based on where their hubs are.
Semi-hypothetical example:
United and Delta compete on flights from SFO to STL.
United has a hub in Denver so this is an efficient connector for them. Denver is right in the middle between SFO and STL.
Delta's hub is in MSP. This awkward connection costs Delta more to operate and is less desirable to the consumer. To stay in the game, Delta might price its flights to STL less than the United flight even though it costs them more.
Meanwhile, Delta has a non-stop flight to MSP. Customers like that so Delta may charge a premium for SFO -> MSP above what United can charge for their connector flight. And that SFO -> MSP fare just might be more than Delta's SFO -> STL fare.