Why do some people have bulging veins on their arms?

Generally speaking, what you are referring to is termed "vascularity" if it is reference to healthy subcutaneous blood vessels. Body builders get those from larger - than - average musculature (which requires increased blood flow) and reduced subcutaneous (under the skin) fat.

On the other hand, a sedentary person who gets pronounced surface blood vessels may well have varicose veins or an equivalent malformation of the blood vessels due to their lack of physical health.