Because the time/effort/ cost of water changes, dosing, and maintenance is so little in that small of a tank. A 50% water change would cost you $1 in a 10gal tank, for example.
There really isn't much room for a skimmer, reactors, fuge, etc anyway.
I agree that extra water volume is generally good, but the juice isn't worth the squeeze on such a small tank, in my opinion.