Scientists do a lot of grotesque things to mice, rats and insects all the time. We have to test stuff on someone, and humans are not really acceptable, even with consent.
Isn't a consenting subject better than a forced subject? The mice had no say, but a human does..
There are other reasons why test animals are not that useful, mostly because they aren't human, and what works in them might not work on people, and what doesn't work in them won't be tested on people.
Over time animal testing has been decreasing and will continue to decrease while cell cultures become easier to create and closer to the cell structures that need to be tested, but for now they are needed in many fields of science.