There are several reasons that waiting for results is not preferable... One, if the talent is from out of town and were traveling to shoot, then they'd either have to travel twice, or be housed in the interim. This could be expensive.

Secondarily, if you're having to wait for results, you would need to keep the talent under lock and key to make sure that they had no sexual relations during the wait period. If they did, that would compromise the test. Taking the word of a model in regards to them not having sexual relations during the wait time... is the same as accepting their word and not having a test done at all.

In the advent of swifter, more accurate testing, these issues are fading into the background - it's becoming more of an issue of whether the studio is willing to make these sacrifices or not, for whatever reasons happen to motivate them.

I don't work for a productions company, so I'm not versed in costs of filming, travel, model payments, equipment and the like - but what I mentioned above are the problems that I would logically find in having to wait for a test, there may be more that are beyond my kenning, but those are two that stick out the most to me.

Kindest regards.