The Master asks if all present are Entered Apprentices or Fellowcrafts when the Lodge opens on the First or Second Degree respectively, so why does the Tiler have to be a Master Mason?

The short answer is because all elected and appointed Lodge officers must be Master Masons and that’s what it says in the Ritual. The longer answer is that we don’t stop being Entered Apprentices or Fellowcrafts just because we have had the Master Mason Degree conferred upon us. The Master Mason Degree embraces the Fellowcraft and Entered Apprentice Degrees, and the Fellowcraft Degree embraces the Entered Apprentice Degree. Thus, a Master Mason embodies all three Degrees. A different way of thinking it might be if the Master asked whether all present are in possession of the secrets of Entered Apprentice (or Fellowcraft as the case may be)—but the effect would be the same.