Why are all the deps bundled?

[brosasaki]

Hello,
I'm in the process of trying to package Flycast for GNU Guix, and one of the principles of packaging for that distro is to un-bundle bundled deps when possible to reduce resource usage and allow easy security updates. However, Flycast seems to bundle almost all of its dependencies as git submodules. Why is this so?
If it's simply out of convenience to users? Or, does Flycast depend on the specific versions of these libraries that are bundled, or not-yet-upstreamed patches?
If I wrote a PR that has the build system use the build environment's installed libraries if possible, and the bundled libs only otherwise, would that be accepted? Or maybe outright remove as many bundled libs as possible, and leave assembling the build environment to the packaging tools (github actions, etc.)?
Sorry if any of this seems ignorant, I'm not a professionally trained programmer and I'm not sure what the conventions are.