There's not enough incentive to train it currently. No manager in their right mind is going to spend half a season getting his players FT from average to prominent and seeing their FT% go from 60% to 67% when they could get their players OD increased to sensational from proficient/prominent or JS increased to prolific from strong.
Of course, if you have crappy stamina, those numbers can dip significantly, but this is definitely not a problem with free throw shooting.