Why are whale sharks sharks not whales?

1) Whales are mammals and do not have gills, they come to the surface to breathe.

2) Whale Sharks have gills and as other sharks receive oxygen through the water.

3) Whales sharks have the same cartilaginous skeleton structure as other sharks

Tthe reason its called WHALE shark is because it is the largest of all the sharks, and as whales are the currently biggest mammals on the earth it only seems appropriate.
No. it isn't a reptile.