Apt is not the package manager for most of those platforms. Thus an operating system package manager-based approach would not work for everyone.
That said, several operating systems do "vendor" pip packages so that apt or the equivalent operating system package manager can install a known good version of the package. This approach can introduce problems, but it also can prevent problems depending on the operating system and programming language.
Because not eveyone uses an apt-based package manager.
Python runs on quite a few platforms:
https://github.com/python/cpython/blob/main/Lib/platform.py
Apt is not the package manager for most of those platforms. Thus an operating system package manager-based approach would not work for everyone.
That said, several operating systems do "vendor" pip packages so that apt or the equivalent operating system package manager can install a known good version of the package. This approach can introduce problems, but it also can prevent problems depending on the operating system and programming language.