They sometimes use the IP of others and it can be a real headache or impossible to get permission from everyone.
This argument seems hollow, releasing source code is not an all or nothing situation. They can just release what they are allowed to, and let the community replace the missing stuff.
Releasing anything is better than releasing nothing and letting the community reverse engineer everything instead of just some third-party libraries.
But also, in a world where such a law did exist, it would naturally force every third-party to create their contracts in a way that would allow the eventual release of the source code, or lose out on the deal and subsequently, the money.