Because (even if your hardware actually supports it, which is more than just unlikely) it's not allowed in your region. The limits as imposed by the regulatory domain database are absolute, as measured at the antenna, this means they include the antenna gain (which is somewhere in the 2.5-3 dB range for most routers) - so you can 'only' set the card to emit 27 dB, as the remaining 3 dB are coming from the antenna - with the total emission being 30 dB (again, that is assuming that your card really can do 27 dB, which is unlikely).