…because that is after all comprehensively forbidden on multiple layers of the software stack. That of course makes it harder to tinker with the system, and that sucks to some degree.
With v256 we are thus opening up things a bit: even though the primary focus of these tools (and of our general idea of what an OS should look like) is true immutability, there's now a new --mutable= switch to systemd-sysext/systemd-confext.