Most companies never release their source code. It'd be sort of ridiculous to expect one to release the source to their games while they're still active.
Revolution and AdventureSoft have both released code to ScummVM, and both are still in business. Other than that, Passfield Games have managed somehow to retain the rights to the old Binary Illusions game FoTAQ and released source to ScummVM. The same goes for Discworld where even if the original company is out of business, Terry Pratchett himself managed to find a way to authorize a source release.