One of the major fears every company faces today is patent trolls. Regardless of whether you are in the right or not, a patent infringement lawsuit will be an immense cost to fight. Opening up internal source code or having public record of which tools/libraries you use by contributing back to them significantly increases the attack surface for patent trolls. Many companies, like Netflix, are willing to take on that risk. Many other companies are not willing to take on that risk.
(Also not a lawyer)