You seem a little unsophisticated about what's important when actually using Linux, I changed the post tagging from "STM32 MCU" to "STM32 MPU", as you don't seem to understand the differences, or expectations.
So perhaps rather jumping straight into "WHY?" questions, perhaps step back, and consider the obvious differences you might see when comparing a simple micro-controller based board to one running a PC, tablet, or phone, hosting a full blown operating system.
To run Linux properly, as intended, you need an MMU, a lot of memory, and the ability to visualize that memory into relatively large contiguous pools perhaps up to 4GB, and hiding the memory you see from those other apps have visibility into. Linux applications expect linear memory, not partially filled slices of 256MB holding ROM, RAM, FLASH, PERIPHERALS, etc. Nor peripherals your app randomly initializes or takes full control over.
Not only is having a lot of RAM important, but also eMMC or NAND memory to accommodate the image files, and a file system holding applications and data.
The system design, and build processes are significantly different. It's a paradigm shift, and there are certainly groups within ST that have been doing Linux for decades.
