Because if you can't do `docker build` locally when learning or building software or studying IT then you will look for alternatives which allow you to achieve the final goal in a similar or "just good enough" way and then you will never start using Docker, not even in production.
For many developers Docker alone has a huge cost of entry in terms of learning. If you also ask them to pay for something which many dread to learn then even less people will adopt a technology which is actually one of the best innovations in software delivery from the last decade.
For many developers Docker alone has a huge cost of entry in terms of learning. If you also ask them to pay for something which many dread to learn then even less people will adopt a technology which is actually one of the best innovations in software delivery from the last decade.