There seems to be a lot of confusion around free software and open source. You can sell open source software (as a matter of fact, you can sell free software too). First of all, open source only means that source is available - the license is up to the developer though and it might as well be paid for commercial use etc. "Free" has two meanings - "free of charge" and "free as in freedom". I'm not arguing for "free of charge" - while it's nice to have software that you don't have to pay for, I could imagine, say, buying Microsoft Windows if I had access to the source code and could hack it as I go.

"free as in freedom" means that I can not only access the code, but also redistribute it and use it in any way I see fit. This doesn't mean that the developers can't sell it though - they can sell support, extra features, hosting and so on. There are ways to profit from free/open source software. I could as well imagine Gitlab Foundation and Gitlab Ltd, where one provides the core and the other non-free modules that cover enterprise use cases. I don't like their current scheme though.

There's no reason for any software not to be open source, aside from greed.