Because x8 is simply shifting 3 bits left so it's "free" (fixed-sized shifts are very simple to implement), whereas generating the x7 would require the addition

Because x8 is simply shifting 3 bits left so it's "free" (fixed-sized shifts are very simple to implement), whereas generating the x7 would require the additional circuitry, which additionally will be more expensive than the x3 was (since it's another shift+add beyond what x3 required)