A carbohydrate is not a simple organic structure, the synthesis of molecules like that is quite complicated, and involves a lot of inefficiencies and filtering out of by-products.

Chemical fuels like methane or ethane are far simpler, and just inputting energy and synthesising them from basic building blocks is difficult to do cheaply and at scale.

It's expensive and it tastes awful. It's hard to give an easy answer to "why is this super complicated thing expensive?", and I'm no expert, but you essentially have to replicate the specialized microbiological machinery of multiple plant species.