Because it is a filter feeder. The whale shark's gills have two functions; in addition to extracting oxygen from seawater the gills filter tiny planktonic organisms on which the shark feeds. When a whale shark is feeding it pumps large volumes of water over its gills and out of it's gill slits. The shark's gills have very fine gill rakers which function as sieves and strain plankton from the water.

Whale shark gills start at end the of the pectoral fins on both sides.
There are 5 gill slits containing 5 gill arches.
All carpet sharks also have a Spiracle, this is an aide to respiration and used to be a gill.