Because while they enable A to sell B's flights as their own, they do not necessarily (depending on the agreement/flights in question- in fact almost never) have the same cost/revenue structure of it were A's own flights.

While oneworld has slightly more liberal regulations (mileage accrual according to marketing carrier/class, lounge access when on EK metal/QF code for ow elites- a QF exemption, QF lounge access when on JQ code/metal-for unknown/unpublished reasons etc), the guiding rule is still 'oneworld marketed & operated flights'.