Why not?

If you input $1000 into process A which returns $20, and inputing $1000 into process B returns $30, you'd be insane to invest in process A and not process B, right?

That example only says 3% margin is better than 2% margin, not whether the hypothetical process yields better results than a bond paying 4% (or whatever). If the said process takes exactly 1 year to complete, and requires all the inputs to be provided upfront, then its margins can be directly compared to bond yields, but businesses are rarely that simple.