Why sharks drown in the water?

Not all sharks drown in water. That "fact" is a myth based on over-generalization from a small number of examples. Only about 24 species of the 470 species of sharks must move in order to breathe. That's like 5%.

Sharks have a modified gill slit just behind the eye called the spiracle. All the elasmobranchs have it.The spiracle plays an important role in passing water over the gills.

SOME sharks must keep moving to pass water over their gills for gas exchange (called ram ventilation). Those few are among the large pelagic sharks whose general level of activity is such that buccal-spiracle breathing cannot provided enough O2. Obligate ram-ventilation is also true of some pelagic bony fish such as the large tunas.

So 95% of sharks do NOT have to keep moving. Here are a few that are perfectly happy not swimming and breathing just fine: basking shark, reef white-tip, nurse, pyjama, bamboo, frilled, grey reef, wobbegong, port jackson, leopard, dogfish, horn sharks, etc., etc.

Well sharks can't really drown in water, because they are fish and have gills. They can only drown in water if they stop moving because sharks need to move and swim so they can push water over their gills, and by doing this the shark breathes in oxygen.

They can drown when they stop moving due to the fact that they need to move to push water over their gills which in turn creates oxygen.

if they stop moving they don't get enough oxygen so they drown

They stop moving