They were depending on you not knowing, and that is never good for you

It's most often done for the sole purpose of retaining the ability to more easily profit off of your work. When you open source your software you are basically taking the most straight forward profit model off the table. Some projects do of course manage to still make it work, but only when the user base is composed of tech-savvy enthusiasts. If you're open sourcing a desktop application targeted at the average user, like a game. It's never going to work unless you hold something back (e.g. art assets).

We need to stop with this false narrative that developers choose to keep their software closed sourced for malicious reasons. The truth is that profiting off of FOSS software is inherently difficult.