blender pip installs to user folder in python path
but by default blender ignores the python path, causing the installed module to not be found on startup

i noticed that blender_pip_addon adds site_dir to the path. which solves this issue, but only if this addon is enabled.
in this PR i suggested to remove this behaviour
proposal
A better solution would be to install to the blender installation site packages, or at least give the user the option in a setting screen to do so. This will result in a clean install where modules are found as expected, without changing the environment for blender.