RPi is a ton of computing power that you can shove almost wherever you want for low cost. Even the Pi Zero ($5) has 512MB RAM and an ARM11 running at 1GHz. It has HDMI & USB.

I’d say that your criticism could be seen as “why use a microcontroller when you could just use a PLA?” Well, there are a lot of reasons why you might choose a PLA, but microcontrollers are cheap these days and easier to program. And the Raspberry Pi is also cheap and easier to program than microcontrollers.

The Pi is easier to program than a microcontroller, more powerful, and still quite cheap. Cheap enough that it’s sometimes being kept in “production” designs.