I don't know, they just never did. Thin eyebrows were really in when I was a teenager, so that's what I did. I thought it looked cute back then haha. I think constantly plucking the hairs over a long period of time can damage the follicle, like traction alopecia. So in time this can cause the hair to not grow back. I think this is what happened in my case.
Get eyebrow transplant. Hair transplant surgeons can easily do that.