Drug reverses age-related cognitive decline within days

No, they are used because they are cheap, reproduce fast and are legal to experiment on.

For mice results in the top journals we're at about 1/3 applying to humans, which includes "has the effect and also starts killing you" if I remember correctly. And that's with the enormous selection bias of only counting the stuff in Cell and Nature and similar.

Still not good models for human diseases. From the article you linked

“ Regarding experiments on mice, some researchers have complained that "years and billions of dollars have been wasted following false leads" as a result of a preoccupation with the use of these animals in studies”