there are a number of things that stacks the odds against the larval clown.
first larva clowns are attracted to light, which in a reef tank puts them promptly on the dinner menu
it also puts them up in the water column where they can get sucked into overflows, pumps, and anything else that can smash/cut/eaten
then you need food, the ocean does have a lot of food, there are many strands of algae that fit the bill, then the zillions of other critters like copepods, ciliates, clam & oister ?larva? and eggs and ....
there is a reason that the ocean isnt orange with white stripes, not many make it in the wild either.