I think the issue is that they wanted to first identify and test the idea first. If you add in humans, you have to find volunteers and then waivers because no one is really willing to hang around in the nude and get bit by horseflies for a day.

As well for humans, you will have to consider demographical data and other data like gender, height, weight, body fat, in general how healthy the human is etc. which lead to even more questions.

Using just the models can mitigate these concerns