Me neither. It doesn't make sense to me. Of course the fact that they can even brew without opposable thumbs is astounding! Dogs, however, are on the other end of the spectrum. They only like to use a bucket once and throw it away. That doesn't make sense either.
Or just reduce your batch size a little bit - reduce every ingredient by 20% and you have a 4-gallon batch recipe that will fit in your five-gallon bucket with plenty of headspace.
I'd use those for utility buckets, but not to ferment in. Go get a 6.5 gallon fermenter bucket that has a lid that's set up for an airlock. They're cheap.
If you ever do BIAB, you can use two of those buckets for squeezing the grain bag. Drill a bunch of large holes (mine are 7/8") in the bottom and lower sides of one of the buckets, put it inside one with no holes. When they "nest" there is a decent space left at the bottom for the wort. Use a fry pan lid (or another bucket) to press down on the bag. Works great.