For the same reason propellers aren't shaped like compressor blades inside a cylindrical encasing. The cylindrical encasing dramatically reduces air flow, but i

For the same reason propellers aren't shaped like compressor blades inside a cylindrical encasing. The cylindrical encasing dramatically reduces air flow, but is necessary for large pressure changes. For a brayton cycle heat engine, like a jet engine, you need to maximize pressure gradient for optimal efficiency, but if you're not burning fuel then you want to minimize pressure gradient and maximize mass flow rate.