What is stopping you from selling projects (instead of your time) as a freelancer? E.g. you agree on a price and date for you to deliver software X with feature

What is stopping you from selling projects (instead of your time) as a freelancer? E.g. you agree on a price and date for you to deliver software X with features A,B,C ? If you finish the job quickly, you can run multiple such projects in parallel, thereby earning a multitude of what someone slower would earn.