It's a package manager in the vein of "old-school" package managers that came from Linux distros and whatnot. It maintains a global dependency chain across your entire machine. This can be good for security fixes in that you only have 1 copy of a package & everyone references it. This is not good for development because it doesn't provide a sandbox'ed environment for you to do development in (ala cargo as others have mentioned). It also causes issue if you try to install 2 packages but they rely on incompatible versions of a popular package meaning you have to choose which package you want installed.

Some of this has been mitigated with virtualenv but having a project express it's packages & have that automatically reflected in the environment.

Finally, Cargo to my knowledge actually lets multiple dependencies exist (even within the same project!!!) so that you can have a dependency like:

                        dep1 -- dep3 <= v1.6
                      /     

< my awesome project> -------- dep3 >= 3.0 \ dep2 -- dep3 >= 2.0

That's not possible if you don't have the right language hooks because module resolution needs to be aware of the version of the library (i.e. when you go `import numpy`, it actually needs to be aware of the package it's being imported from to resolve that correctly).

Now whether or not it's a good idea to support this kind of dependency stuff can be controversial. In practice though clearly it does cause problems the larger your codebase gets as you're more likely to have some nested dependency chain that's time-consuming to upgrade so you'd rather move faster than make sure you're only running 1 version of the dependency.