Why quadruple, anyway? I've seen quite a number of ~$200 laptops and found their screens and batteries tolerable (although, obviously, not really good).
It's a matter of scale. $200 laptops are great if you can produce thousands of them and have a market willing to buy them. That's why Chromebooks sell so well. Let's also not forget that Chromebook/Android devices enjoy the subsidy provided by Google's advertising division.
I doubt the Pi 400 with a screen, battery, touchpad and integrated webcam would be able to sell in numbers that would justify the manufacturing and distribution costs.
