Because humans are more valuable. If you had to choose between saving one human, and one hundred rats, which would you choose? We test on rats until we deem it safe and ethical enough to progress to testing on humans.
Can you answer the question, "If you had to choose between saving one human and one hundred rats,which would you choose?" The answer to your questions is related to this one.
What argument? I haven't made an argument, I want to know your position and what it is about humans that makes them more valuable than non-human animals.