Rug pulls, forks, and open-source feudalism

Building the software you rely on from source by default is one way to reduce the impact these events have on you and shift the power dynamic. If you're installing binaries/images from a vendor (free or otherwise), transitioning to a fork may be an undertaking and a sweaty risk-assessment.

Switching your existing build-infra to sync sources from a new remote should be a snap.

Also no major need to hound maintainers to ship a release or merge that neglected bugfix or feature you desperately need - just cherry-pick it.

Depends on the actual software licence, many commercial vendors do provide source code, however the licence doesn't allow you to do whatever you feel like with code, even if technically it is possible to do so.

This happens a lot in commercial products where scripting languages are used, for example.

Or enterprise consulting as another example, where the code is delivered as part of the project, but it is bound to the agency for compiling purposes, unless the customer pays extra for that right.