Because the landing techniques and skill for operating a float variant are different from a land variant in amphibious mode.
Hmmm, Bose...
A pilot could be qualified and suitably experienced to fly a C206 wheel plane, we all agree about that. That same pilot could then continue onward to qualify and be rated to fly the C206 as a float plane. In doing that, that pilot might never fly an amphibian version of the 206. However, As I understand it, they would be entitled to, by virtue of having the land and sea ratings associated with their qualification, to fly a 206 amphibian. Is there an additional "amphibian" rating to add to the land and sea ratings for those types (commonly single Cessnas) which may be configured as amphibious floatplanes?
I would be delighted to think that amphibian wheel plane flying privilege was considered worthy of the training in its own right, as the amphibian version of any wheel plane is very different to fly, and warrants additional training. In Canada, these fine distinctions are not made in licensing. (We don't even have a tailwheel endorsement!). It really becomes the insurers who dictate that a pilot might need additional training.