Why Biobutanol & not Biohexanol

I thought the carbon BONDS are where energy is stored. So why are
we taking SIX carbon glucose or cellulose and producing TWO or three
carbon alcohols as fuel? Isn't that what makes octane so special? Am
I missing something? Aren't the microrganisms using up more energy
than an industrial process would take for the RIGHT conversion?

(Forgive me but I took Orgo in 1980-81 and didn't do too well in
it. I actually did great in high school orgo, but after three years
of engineering courses, memorising orgo just refused to happen. I
should have taken orgo earlier.)