Because diagnosing an animal concussion is harder than diagnosing a human concussion. Animals can’t tell you if they have a headache, light sensitivity, nausea, etc. You would only be able to diagnose the worst class 3 concussions where the animal can’t even walk straight. It’s probably not ethical to subject animals to that much pain if you can’t even confirm half of the injuries.

Aren't concussions visible in MRI?

How cruel.

Cancers are regularly induced in mice and rats. Almost every single medicine has been tested on animals before they're used on humans. Modern medicine is built on animal models. It's unfortunate, but a sufficient tradeoff a vast majority of people are willing to make.

I’m not entirely sure what you are saying. Is it not cruel, or that the cruelty is OK?

They're saying it's cruel, but some cruelty is sometimes needed if the benefits for humanity are worth it.