Why are we missing a 5.5 gallon carboy?

Most winemaker's arsenals have or have available to them carboys from sizes 7.5 gallons all the way down to tiny tincture bottles, however one size that seems to be missing from the list is 5.5 gallons.

We have 7.5, 7, 6.5, 6 then it goes to 5

For people who target 5 gallon batches and step down in sizing rather than adding water to back-fill, 5.5 gallons seems to be a missing step from the progression. Why hasn't this gap been filled?