[crabbone]

> Why not just say "pip install idp-engine"...?

(I have nothing to do with IDP-Z3) This is the general pattern:

* If you are a developer: get sources and work on sources, because, probably, you want to add to them or modify them etc. Part of this experience may be creating a package and installing it with pip or some other package installation program, but it's not usually what other developers (contributors) would be working on.

* If you are a user: use the supported package manager to obtain the package and use it through documented interface.

I'm honestly not sure if this is why or how these instructions came to be, but this is a fairly common thing to do, so, w/o looking, I'd guess this is what it is.