There's no incentive to do so.
It takes time and money to ensure you're not releasing proprietary code you don't own. It likely stops you from ever re-releasing it, even if that's very unlikely.
I understand that binary libraries can be owned by a different company. But if you strip all that could the resulting code still be infringing the copy rights of third parties? I never worked at a company where someone else's source was integrated to our tree so I can't tell.
For game engines it's actually fairly common to have source access but not a redistribution license.