It is never as simple as throwing it over the wall. It's one thing to make code available for review, and another to release a package that recreates a functional product.

Companies may have elaborate infrastructures and sets of dependencies, and it may take a lot of time to come up with something that even builds correctly in isolation.

Open-source basically has to be planned early on: use freely-available tools, use common platforms, etc. and then open-sourcing it later may not be too difficult.