I was under the impression that Arch and its derivatives was made for power users. I stand corrected.
As a developer, I have my own package dependency tree. Packages that I never want to put under a Pacman umbrella, but which are still common enough for justifying being globally accessible.
As long as the system ignores /usr/local, there is no rationale in the world that justifies pip not installing to /usr/local. This is the whole reason /usr/local have come into existence. And with more than 30 years of Unix experience, I've still yet do discover any serious problems using /usr/local.
But there is, of course, a solution for us that gives Arch the big middle finger for the stance on /usr/local.
You can always create a $HOME/.config/pip/pip.conf containing:
target = /usr/local/lib/python3/site-packages
...and also prepend this path to PYTHONPATH. Voila!
What I just have to add, is that allowing pip to install to /usr is infinitely worse than anything. This is allowing the system to be broken.