This is fundamentally mistaken advice, not because it’s necessarily it’s wrong to run python modules in the administrator context (although you probably don’t need to) but it is always a bad idea to run command with root/administrator privileges without understanding why you are doing so and why root privs are needed.

If you don’t need global system access to these modules, appending —-user to pip will install in the user context, no elevated privs needed.

Why isn't it the default?

According to the superuser link, in the previous version it worked by default without privileged access.

I obviously run pip as administrator... before replying you should try it yourself.