Yep. To brew 15 gallon batches, you basically need an 18-20 gallon fermenter. And a 20 gallon brew pot. And *minimum* 15 gallon mash tun, while 20+ would be better. And then a bigger HLT. And bigger burners to ensure you can actually bring the sucker to a boil. And pumps, because nobody but superman is going to be able to lift those weights, particularly for some of the hotter liquids. And a counterflow or plate chiller, because an immersion is going to take years to cool 15+ gal of wort. Oh, and if you want more than one beer on tap at a time, you need a MUCH bigger kegerator because the 15.5 gallon Sanke kegs are so much bigger in diameter.
All that equals huge $$$ -- and most homebrewers are interested in variety. 15 gallon batches take a LONG time to drink. I brew 10-gallon batches, and I usually have too much beer in the house because I want to brew often enough to have a good variety, but the amount of time to actually drain a batch is many months because I'm serving 5 beers in the kegerator at a single time and I drink whatever I'm in the mood for, so they all last a long time.
Can it be done? Of course! Hell, there's a guy in my homebrew club who brews 50-gallon batches. But they're the exception, rather than the rule, and for good reason.