Yes that's a good question for the researchers why they gave 5mg/kg fina/day.
But if we look at DHT impact, even small amounts give the same effect as high dosages:
This is serum DHT inhibition, it's only modestly correlated with what's happening in the tissue, which is where the majority of DHT is produced. Serum DHT is a small fraction of the total DHT within the body. If you look at the studies regarding the concentration of finasteride in semen, the maximum seen is in those taking proscar (5 mg) and it amounts to 21 ng/mL (
https://onlinelibrary.wiley.com/doi/full/10.2164/jandrol.109.009381). Even if you quadrupled the amount of semen required for something like hyperspermia, the total amount of finasteride in the semen would still be negligible.
Anyways, that's besides the point. With the extreme dosage that was administered to the rats in this study it's impossible to tell whether or not the same experimental outcome would be observed if realistic dosages were used. An equivalent dose in a human male having a mass as low as 45kg would still be 225 mg of finasteride per day. At dosages that high, how can you be sure that you're not achieving high levels of saturation and increasing the concentration of finasteride in the seminal fluid to the point where it would be high enough to observe the effect reported in the study?
If they designed their study in a way where you had three groups of rats in which the following conditions were met:
Group 1. Rats administered the "rat equivalent" of 5 mg of finasteride per day by mass, this would be somewhere around 0.025 mg depending on the species.
Group 2. Rats administered the above amount of finasteride per day, then taken off the drug for a period of 1-2 months.
Group 3. Placebo group
You could also add additional groups that take the rats off the drug for a variable amount of time to get an estimate of the required time off the medication to eliminate finasteride in the semen.
Then you would be able to answer:
1. Do realistic dosages of finasteride cause said defects in the offspring of rats treated with finasteride?
2. Is the effect permanent, or is it only during treatment?
Unfortunately, the design wasn't set up in that way, so this study doesn't tell finasteride users much.