you answer the why with your thread title: they’re system apps.
Default apps are system apps historically because LineageOS didn’t need a package distribution system, so /e/OS didn’t have one either when starting out. “temporal solutions become permanent”.
By now everybody wants that capability including Murena, but it isn’t trivial and didn’t get priority yet over product features, because - my outside guess - it just isn’t as visible.
The demand is acknowledged in the 2020 and 2021 roadmaps and other posts. The lowest forum id thread that asked to remove default apps is from 2018 and many, many user threads since. Technically things are still pretty entrenched.