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.
What a weird article. Makes sweeping conclusions like "women are fine with trans" issues, casually ignoring the ~50% that responded unsure or negatively.
The numbers are generally evenly split positive-to-negative and whilst, yes men are between 5-10% or so more likely to respond negatively, phrasing it in a way where you treat men and women as monoliths isn't very helpful to addressing the goal of trans positivity.