They're human specific ailments. We create a fake version of them in mice, then we fix the fake version. The basic problem with these issues is we don't understand the root cause. So we can replicate the symptoms in a mouse model then fix the symptoms, but that doesn't work in humans because the root cause is still there.

I guess it's because most major disorders and diseases have so many pathways at play that figuring out which one's actually causing the problem at the individual level is just too tricky.

The other thing concerns how potent the effect is to be therapeutic. In many cases, the effect is just marginal to be meaningful.