Even $300 is a big premium for what you get. Basically just an extra camera and a LIDAR sensor that nobody knows what to do with yet. Oh, and a shiny stainless-steel band around the edge.
I think what Apple has realized is that a certain subset of their customers will always want the best model, no matter how little sense the price makes compared to the regular model. For some people, it will always be a status symbol (or an enthusiast's obsession). Or they're of the economic class where $300 just doesn't even register on their budget. As one reviewer put it, the 12 Pro is "the shiny one", and for some people that's all it takes.
What this amounts to is that these whales can subsidize things like the $400 SE model, which is an incredible value. And I'm completely fine with that arrangement personally.