One of the key differences is the company has "skin in the game" at an interview, but not in the 6-hour project.
When you interview, you know the company has included you in a shortlist and they will put time into getting to know you. For the time put in, you have a chance to pitch yourself, to at least get across your general abilities and personality, and some people will pay some attention. Also you can learn more about the company and the people you might work with.
When you get a take-home project, it's like the way spam is cheap: It takes almost no effort for the company to send the same project to a lot more candidates at low cost.
You do the work, but there's no guarantee the company will really assess it. Sometimes the company drops the ball and just "forgets" the application, or discards it the same way résumés are filtered: By skimming though 6-hour project submissions and casually throwing some of them away. It's not uncommon to not hear back anything from a company after sending the result of working on their project.
Some candidates will enjoy doing a project more than an interview, but that's not the only consideration in determining their value to the candidate.
Personal experience: I have never had an interview last more than an hour. I have also been hired twice on a "just trust me" model and it worked reasonably well.
Replacing a day of interviews with a 6 hour project sounds fair. That isn't how most companies use projects.
It always seems strange to me that people won't bat an eye at taking a day (or more) off work to come on-site for technical interviews, yet the idea of giving someone a 6-hour project do at home, at their leisure, is somehow a bridge too far.
In real-world hiring experience: I gave candidates a choice between on-site interviews and take-home projects for a while. Zero people chose on-site interviews when given the option, so I stopped offering it.
The bottom line is that interviewing requires time. If someone isn't willing to lift a finger or allocate a few hours to an interview, how are we supposed to gauge their competency? The "just trust me" model doesn't work at all.