That would probably quadruple the price. Look at PiTop. They used to include batteries and a display, but have moved away from that, as these are the most expensive peripherals.
You can by a cheap display, or use your existing TV. A USB power supply isn't too expensive either.
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.
I have indeed got enough spare displays but I just want to carry my Pi 4 around and use it in public transport and everywhere so a separate display and a web of cables doesn't seem particularly convenient. Perhaps I could buy a PiTop but their current model is not a laptop.
