Because there is still so much we don’t know about biology in general, we need to study the real thing. Since working with humans is usually not really an option we have to go for the next best thing. It’s incredibly well regulated, you can’t even just have a rat in the lab and just watch it without tons of paperwork and careful consideration by a committee.
I’ve done quite a bit of animal work myself and I very often got negative responses from people. What most people don’t know is that researchers are extremely concerned with animal welfare. More so than most pet-owners. I’ve always found it weird that researchers get the bad rep while exterminators get off scott-free.