I think this is the real reason. You either have a scheme based purely on revenue (e.g. you get Silver if you spend £10k+ per year on BA, or something like that) or on something like miles/TPs. The latter enables leisure travellers and others who actually make their own travel decisions to take advantage of various offers and still earn status; in other words, it rewards people who actually plan their travel in a way that takes advantage of the scheme rules.

The OP is trying to have the best of both worlds. If BA allowed this, it would become a great deal easier to get status, because there are a lot of cheap TP-earning fares out there, but some are a good deal of hassle to use (for example they involve flying between two cities on another continent). So if BA allowed this, it would have to recalibrate its TPs, either by giving fewer TPs on certain routes or by raising the TP threshold.