I wonder how much of that is due to dealers, who want to upsell. Do they even keep the base model in stock, or does it have to be special ordered (or today, we can give you a "discount" on the fancy model that still has a higher profit margin for us).
I'm just speculating; the same reasoning wouldn't apply to the iPhone mini. But car dealers have a lot of incentive to skew the results. It takes a fair bit of willpower to say "I am buying this specific car I want and will go elsewhere if I can't have it."
Anecdotally: I helped someone look for a cheap car recently and the base models are everywhere on my local lots. YMMV and I'm sure someone will respond that their area doesn't have any, but in my recent experience it wasn't hard to find a base model at all.
It's driven by consumer demand: If you can pay $30/month on your 5-year loan and get heated seats and a nicer navigation system, that's $1/day for 5 years and then you own it. It's easy to talk yourself into stepping up to something nicer that you're going to use every day.
Just as an anecdote - When I was buying my last car I went in and asked for just the base model with nothing added onto it, ie not even the "eXtra Special" designations, and the dealership said they probably won't have any for a long time and if I'd like to pay 50% more for one with some features added on.
I declined and kept looking at the inventory of the 4-5 dealerships nearest to me. For six months they never had a single base model.
I started looking at another maker and they seemed to have base models that just wouldn't sell, stuck on lots for that same time period.