We get asked this question at work alot and the simple answers we give are..
- The tanks in the shop have an automatic water changer that comes on twice a day. NO ONE will do this on their tank at home...
- They are gravel cleaned usually twice a week to remove all the poo....Most tanks are lucky if it gets done twice yearly!
- And most of all its completely temporary, a tank of fish might be half emptied within a week, so even if you have 250 cardinal tetra in a 20 gallon tank, by the end of the week there may only be 50 left, and within 2 weeks the entire batch will have been sold. In your tank the fish are in there for the long run so you can't cram the tank full of fish and hope for the best.
Ben