The male performers obviously have to have control over the extent of their arousal, providing they have any at all.

While some of the female performers are really attractive, doing their part as the infamous paid prop many days a week and sometimes more than once a day possibly just gets them accustomed to situations people outside the industry seem to envy.

So being not aroused enough to ejaculate is better than not being able to stay erect and called a limp dick. Maintaining the balance here while performing is what they are paid for.

Finishing the money shot on their own might also have to do with designated target areas the director prefers or demands.