Again opportunity costs. It almost always makes sense to spend the money on the most efficient means to achieve the goal. Money spent paying farmers and ethanol refiners to inefficiently produce 25% lower carbon fuel could instead be directed at other endeavours that for the same cost reduce carbon emissions more.

The difference is we already grow corn at scale beyond market need. Probably less has to be paid in that effort than starting up some other industry. Which still can be done along side the corn shouldering the load until that industry reaches the scale of the corn industry's waste product.