@smileydude Because en emulator is not just a binary you drop there and it runs auto-magically
An emulator may have tons of dependencies, rely on several low-level libraries or event requires specific display systems.
Recalbox should be seen as a firmware: this is not a linux distribution and there is no package mangager. It's an optimized linux built with the minimum requirements. That is, you cannot install dependencies manually.
Obviously, there are several chances that the pico-8 emulator may run flawlessly on Recalbox and I remember an old procedure to install it manually, but we cannot guarantee anything if you do.
And keep in mind, the pico-8 emulator is not free! Even if we whish to do so, we wouldn't be authorized to distribute the pico-8 binaries.
Moreover, you cannot "replace the build-in emulator" as it is a Retroarch Core while the pico-8 emulator is a standalone emulator. There are huge technical differences.