Because feedback in that form is generally worthless. It's like going to a meet-up and tossing around your idea. You'll get a LOT of 'feedback' all of it as completely worthless as what your thinking already.
The whole idea behind a MVP is to get worthwhile feedback about the product. Feedback in the form of customers actions. Will people actually buy this? Will they respond to what I'm doing? Can I get traction?
None of those things can be answered with a submission form on a landing page.
I have long agreed with the author here. While there are answers to be found in dry testing... it takes a prototype to really solicit any truly meaningful answers about a product. At least in my experience.