ive always wondered this same thing myself. walmart and petsmart/petco both stock pacus, another mosnter fish that gets way too big for the people that shop at those stores for fish to handle. another common monster sold at stores are the multiple species of arowna. for thje amount of these fish comming into the stoeres and being sold out, u would think everyone had a 210 gallon tank to accomodate them. other mosnters i have seen sold are, gars sturgeon, parron catfihs, iredecent sharks,and peacock bass. althought there are plenty of people that can indeed accomodate these fish, the amount of people that cannot should be enuff of a deterent for stores to not caryy them, sadly, its not.
ive been out of the hobby for a few years and i bought a 10 gallon tank 8 months ago. and i bought a tiny 2" ornate bichir (another monster fish) to go in it. i shortly afterwoard discovered that this fish would reach 2 feet in time and outgrow my tiny 10 gallon tank. so the next day i bought a 30 gallon for him.he now 12"s long and 3 months ago i bought him a 100 gallon. still too small for life, but itll do for a while.
i myself,just recenlty purchased a florida gar. and yes my tank(100 gallons) will eventually be way too small. but that doesnt stop me from eventually getting him a much larger tank (im planning a 500 gallon for him and my other monsters).
it has always bothered me that the amount of fish reaching extemely large proportions that come into the stores doesnt equll the amount of equaly large tanks that those fish need exiting the stores. but sadly, theres nuthing we can do. its the way the world is and we all have to do our part in educating the new comers in the hobby and preventing them from making mistakes.