Because that is how long it takes. Stop wastching porno movies and think you are supposed to be like the guys in them. Be glad it takes 15 minutes - the longer to enjoy sex.
Dude, some guys can last for minutes while others can go for hours. Personally, if I feel like climaxing quickly, say in a few minutes, then I'll go faster and ejaculate. If I feel like lasting longer, then I'll go moderately and stop before climaxing and then hold back from ejaculating and keep going. Unfortunately, by then my hands are all worn out and I have to stop. God I wish I had a girlfriend! lol.