The composition of GM crops and foods derived from them is carefully studied. Using our knowledge of toxicology, food allergy, and nutrition, it is possible to predict if a food will have an adverse effect based on composition alone. The study of composition is a better indicator of safety than are animal studies on whole foods. Many scientists in fact question if whole food studies in animals are useful and have suggested they not been done. Studies in humans are even more difficult to do and would likely yield little useful information since the diets' composition is the same, the outcome would be the same. Since these are whole foods, with animal studies animals can be fed diets containing large amounts of the food ingredient being tested every day, which would be very difficult to do with humans. Moreover, at the end of a study with animals, post-mortem examinations are performed that allow for a careful pathological examination of most all tissues to understand the pathologies that resulted from consuming large amounts of the whole food tested.
It's a general scientific principle that to do a good animal or human study a clear hypothesis and a means to test it are required. Whole food feeding studies lack an hypothesis. The underlying problem with whole food studies is that foods are complex mixtures of thousands of compounds rather than single pure chemicals. Toxicologists use animal studies with single chemicals to test their toxicity (remember that virtually every chemical including nutrients required in the human diet can be toxic if we ingest too much). Foods are of course generally safe to eat so new varieties of crops are not tested in animals or humans. There is a significant scientific consensus, based on quite a few composition studies, that demonstrates that GM crops composition is more similar to the parental strain from which they were bred than are other varieties of the same crop. So, if we were going to ask for human or animal studies on whole foods, we should be asking for the on crops bred by non-GM methods.