I think that it is because the more you go to the middel of the list PG-SG-SF-PF-C, the more the skills have to be divided over the entire range.
if you take the outer ends, PG and C, they have most single position traingtypes, because they have most specific need of skills...
wingmen and forwards and guards and even teamtrainings do exist as well, and are perfectly okay as trainingtypes.
There isn't any type of training that you would only trani your SF in. ANY type of training you want to give to your SF, is usefull for at least 1 other position on the field, and I think that is why you don't find single position training for SF.
Not saying it is good, or supposed to stay that way, or any other thing one might try to make of my answer. It's just an answer to the question why there is no single position training for SF...
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