Sometimes, it just doesn't make sense. For example, I write and maintain the Linux kernel modules for one of the largest, most popular, and most powerful land vehicles in the world. It wouldn't make sense for us to release these to the open source community because it requires specific hardware that we make, specific FPGA firmware that we interface with, and it will only work with specific software we make. So there's at least one case where it wouldn't make sense to release this, and that's not even getting into the security aspect of things.

A simple answer is that, from a business perspective, closed source is simply easier. It's likely the default way they handle their code.

And at least for GPUs, drivers are pitched as a feature and may contain secrets worth protecting in the eyes of the business.

In addition to the very good replies you already got, sometimes it just wouldn't make a difference.

In some environments, if the code was not developed using a particular process (my experience is with regulated medical devices), it may as well not exist, because putting it into production would be in violation of Federal law.

There are also small niche products where most of the IP is in the driver: lose control of that and you're on your way to being out of business.