TL;DR: I’ve found this to be true in my career. In my experience, whether or not you get a greenfield project depends on your company or the nature of your team.
If courses teach you’ll be building more greenfield projects than not, I’d be hesitant to consider it a well-rounded education in software development.
In many disciplines, a greenfield project is one that lacks constraints imposed by prior work. The analogy is to that of construction on greenfield land where there is no need to work within the constraints of existing buildings or infrastructure.
By definition alone, it seems near impossible to think about starting a true greenfield project. I mean, if you’re building something from scratch you have the ability to pick every set of technology and standards the team will used to build the project.
But most projects start with some type of foundation these days. With some extreme exceptions, most things are going to start at the operating system level or will use a platform like the web and likely some type of framework that sits on top of it.