Because they make money, that's why.
The latest installment of the highly lucrative and action-packed “Fast and Furious” movie franchise hasn’t even hit screens yet and already the studio is anxious for more. Furious 7 will arrive in theaters this April and because series star Paul Walker was tragically killed in a car crash last November while filming was still taking place, audiences will be anxious to see how his character will be written out. Walker didn’t complete all of his scenes prior to the accident so the storyline had to be changed.
Universal Chairman Donna Langley, in a recent Hollywood Reporter roundtable discussion, stated that "We think there’s at least three more (movies)...I think it’s still a growing franchise."
That means, in theory, a "Fast and Furious 10" is entirely possible. However, Langley reiterated that Universal is "very mindful of fatigue." We’ll have to wait and see how "Fast 7" ends to get an idea as to where the storyline is heading.