Because that's what society does.
Transphobes imagine every trans girl as a hulking "super predator" waiting to happen, even when they're a skinny, harmless 15-year-old kid who's been on blockers or HRT and is physically, mentally, emotionally, and socially indistinguishable from a cis girl in every respect save the obvious.
Kids die because of this shit. Because of people like them.
And they're basically okay with that.