Because testing things in laboratories for healthcare requires stringent and rigorous controls. It's a lot easier to standardise animal blood than human blood, and as the comment above highlighted - animal blood is already a waste product, whereas human blood is highly valuable for directly saving lives.
Arguing for less animal usage is a nobler cause, but this is not the fight to pick for it.
Sorry if I'm being naive.
Would you not have the most climate-positive impact by stating in architect and pushing more climate-positive design and construction as much as you can?