Keep in mind that many of these corporate employed developers would likely contribute to the same OSS projects as a part of their job regardless of what company they happen to be employed at.
They're a minority.
Most companies expect developers to focus on what's important to their specific business and not what was important to their previous employer's business. They can't just work on whatever they feel like, especially on company time.
First of all some workplaces won't allow it.
Some companies don't even run on OSS.
Some companies will work on it but not contribute back.
Some companies won't have a budget to fund random / semi-related OSS project development.