I would say it's genetics. Especially if they have brown hair. When my niece came out she came out with a happy trail of dark hair down her lower back into her butt crack. I've never seen that before. And she still has SO much hair.
And my 7 year old is EXTREMELY hairy, but it's blonde. I always look at him like "if this kid was a girl we'd have to shave his legs!"
My dad's side of the family is full blooded Italians, all the men have extensive hair issues. I expect it to continue.
My son is extremely hairy. His issues are absolutely genetic. His upper arms/legs/back are all covered in dark brown hair. It's going to be scary when he's a teenager. He even has a teeny tiny darkish mustache coming in. lol He's 4. His brothers aren't quite as hairy, but all of them have a fair amount of hair on their backs/legs.