The ATS will not engage on the ground until Autoflight is pressed. At and above 400ft pressing PROF will engage the ATS if it is off. My understanding is that PROF can not be engaged on the ground because it is inhibited. Most likely due to the fact that you do not want to be flying an FMS climb profile during during First segment performance incase of an (or dual) engine failure. So you climb at V2+10 until 400 then engage PROF.
BBB