If we could recreate and model humans down to the molecular or atomic level, it might seem barbaric to test medicines on other beings. However, we do not have that level of accuracy in our simulations for the entire human body, nor we do we have close to processing power either.
Also, the point your making doesn't even translate that well. The reason why we can test medicine on animals is exactly because we are quite similar. We have organs that are similar enough to do rudimentary tests.
If we didn't have animal testing, there would be way more dead humans out there. Yes, we do a fantastic job at killing/hurting them ourselves be it indirectly with things like pollution, neglect, and negligence, as well as directly like murder, war (also murder), rape, and so on. But, how many volunteers do you think we'd find to test our things on before even reaching our current phase 1 testing? Would you be willing to give up the life of another human for the sake of another animal? Would you be OK with doing so if it were a human close to you? Would you golunteer yourself?
We are all very good at caring a lot about things that don't directly impact us. Alt-right types are very quick to be anti-trans, devout Christians against abortion and stemcell therapy, and vegans against animal products. However if the trans person is you (or your child), the person requiring aborting to save the mother being yourself, your wife, or your child, the animal being the difference between your survival or your death, then suddenly things get real really quickly.
Animal testing is the same. Sure, unethical testing and rearing without a care for animal welfare is a problem, but if a few thousand mice had to die to save your mother, would you reject the medicine out of principle and also want to prevent any other such medicines being researched in the same manner? Or would you accept it and then scream "no animal testing!!!!1!1!"? Or would you suddenly be OK with animal testing?
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