./configure without `--enable-system-site-packages` will ignore most Python packages on its list of dependencies.
`./configure --enable-system-site-packages` wil try to use the latter, but it might run into a version conflict.
(because most Linux distros don't have the complete range of these available, so some will have to be built from source, etc)
Yes, I find this state of affairs less than ideal, too.
We now also have an option to only build the main component, sagelib.
(but then you'll have to provide the needed dependencies).