Earlier quoted context omitted.
The ethanol is required as an anti-knock additive. The past alternative was tetraethyllead, which had catastrophic environmental impact. https://en.wikipedia.org/wiki/Tetraethyllead#Antiknock_agent
Octane on its own is an anti-knock additive. And ethanol actually makes your mileage go down because it is less energy dense.
Like, compared to even worse carbon chains than octane?
Edit: Never mind got the scale backwards. I'll keep the comment here to shame myself.