Because hair color isn’t homogeneous. Every hair is a slightly different color, and it’s significantly more noticeable in lighter hair. Blonde hairs are predominantly gold, but range from white to brown. In cartoons, gold is also usually just yellow, because animating something metallic is a pain in the ass. Hair is shiny as well, so they picked the average color and called it good enough. Grey hair is the same way.

As for only existing in humans, that’s just not true. Yellow labs and golden retrievers come to mind immediately, and plenty of rodents are blonde. While those are fur, not necessarily hair, golden doodles have hair, and are usually some manner of blonde. Golden macaques, flying squirrels, lions, etc.