The core reason is that they are potentially screwing up a ticket which then has to be used by another airline. If they made some mistake and all the sectors are BA then it's all internal, can be sorted out locally and there are no cost implication. They are basically fearful of doing a bolt on to a ticket which then has a value and requires processing on another airline. Yes it is do-able, if there was a willingness, but experience has shown too many wheels can come off the wagon to the potential detriment of both airline and passenger.