Actually, there are a fair number built, they're just not popular here. Of course, Worksman makes one, but they are very uncommon. Schwinn quit making the old Cycletruck, presumably due to low sales. But a couple of years ago, when I was researching these, I found a number of companies in India still make the Low Gravity bicycles, and I have an Arpan version out in the garage. And I think some Chinese companies do, and a while back, I saw a Brazilian version for sale on the Houston Craigslist.
The Worksman model is a whole bunch cheaper, by the way.
Some thoughts on it. A 40-lb bicycle with groceries is not easily maneuvered around, contrary to the claims on the website. If you have a heavy bike with heavy load, it's hard to pick the whole thing up. If you have a light bike with heavy load in that style basket, then lifting up on the handlebars lifts the rear wheel off the ground, so it can be very hard to lift over curbs. With the load up front, you need the kickstand up front, not in the back like they have it, and the low-gravity and cycletrucks were generally built that way. With different size front and rear wheels, you also have to carry different size spare tubes.