If by "source" you mean the project's version controlled repo, it is for a variety of reasons.

One primary one is, sustainability. The build tarballs are kept forever. Try that with a remote repo that could vanish in a mere 10 or 20 years.. or tomorrow!

And tar is the most stable archive format in existence, with at least half a century of use.

note: Companies that care, should ask themselves what do you do, if you have a build system which relies upon externals, and a part of that build goes down?

And you have an urgent fix to PROD required?

Hope you can find all the bits, unvarnished, scattered on dev boxes? Cobble them together and hope they build, while your PROD is currently borked?

Or do you hotpatch PROD?

If your build process breaks due to an external repo going MIA, then you're doing it wrong.