I believe this happens because it is seen as at best unnecessary and at worst patronising and discriminatory to distinguish a woman's profession because of her gender. There's no such thing as a pilotress or doctoress/doctress. It is on the same lines as using Ms instead of Miss/Mrs - a woman's marital status should be irrelevant in all the same circumstances where a man's status is irrelevant.
Unfortunately most professions are already masculinized, as evinced by their masculine endings and the fact that they used to be the exclusive province of men. Adding a feminine ending draws attention to the fact that this is something a bit 'different' and 'other' away from the 'norm' of being a man. Even 'human' and 'woman' have their roots in 'man' - which is supposed to be a gender neutral term for all of mankind but which, in English at any rate, relates specifically to male people and a subconscious emphasis on maleness as normal and female as 'other'.
I also think it would be a massively uphill struggle to get the world to switch to neutral terms like 'doctum', although where I work no one uses Chairman/woman any more, but simply 'Chair'. Using a female ending doesn't diminish a woman's actual ability, only prejudiced people's perception of it. Far better to ignore any original masculine endings and absorb them as neutral terms instead.