As mentioned, ATHR is not ‘failed’ in direct law. It’s only a technique to manage the pitch/power couple. Have you tried flying with the autothrust engaged in the sim when in direct law? It’s harder than it looks! For a start, you’re having to predict what the autothrust is going to do and you have to keep scanning the engine instruments to anticipate the pitch/power couple, then make the pitch correction, then trim. If it’s a gusty day, the autothrust will start hunting the speed and it will destabilise you potentially even causing PIO.
It is far, far, far easier to set a datum with the thrust levers, disconnect the autothrust and be in total control of the aeroplane. Making as few thrust changes as possible is the key to flying a stabilised approach in direct law.