1) Because Airbus says not to.
2) More importantly, as a/c weight, altitude, temp, etc, can vary tremendously, you don't know that your aircraft will be able to actually sustain flight at the flight path angle you are pitching to. The bird is actual FPA, and what the airplane can fly on one takeoff may not be sustainable on another.
In contrast, an initial target pitch attitude of 15 degrees is considered sustainable, and then you tweak the pitch if need be to fly your desired target airspeed.