That would kneecap public access to anything. Their API is what runs their site, requiring auth for everything would essentially break Reddit for anyone not logged in.
That would kill new accounts. No one would know what was being talked about before onboarding, but without that information, what would their motivation be for onboarding?
Those aren’t always a fit for everywhere. And getting energy from one place to another is an unsolved issue. Just because one option is cheaper than others doesn’t mean that particular option is the better choice. Diversity is very important.