Maintaining internal forks is time consuming and expensive. For big tech companies they might end up saving money on infrastructure but small companies can end

Maintaining internal forks is time consuming and expensive. For big tech companies they might end up saving money on infrastructure but small companies can end up sinking a lot of human resources into the project (especially if you're playing whack-a-mole with upstream changes moving in different directions and breaking things) Open sourcing things increases the likelihood it'll be upsteamed (the use case is more obvious) and increases the likelihood you'll get support (although you now have a community to support). In addition, it's easier to plead your case if upstream breaks something (here's my source vs vague statements about an internal thing at company x)