The US produces a lot of corn, so it is cheap. And corn syrup is a sugar-substitute, so including it in everything makes things taste more addictive. https://en.wikipedia.org/wiki/Bliss_point_(food)
The argument you make though explains why we aren't using algae to produce biofuel. Far more valuable for other purposes as called out in this Forbes post[0]. Here's a choice quote:
> An acre of algae can produce almost 5,000 gallons of biodiesel. It does not compete with food crops for arable land or potable water and could produce over 60 billion gallons/yr that would replace all petroleum-based diesel in the U.S.
[0] https://www.forbes.com/sites/jamesconca/2014/04/20/its-final...