Most likely because of five gallon kegs?I do 3 - 4 gallon batches, stove top, biab, in a five gallon kettle. Smaller batches mean I get to brew more often which means I have more variety on hand. Smaller batches also mean less weight too have to deal with.
You're not required to make five gallons at a time. I've always wondered exactly why five gallons got to be the "standard" .