Well, not really - there are already quite many open source companies and they are doing just fine. The modus operandi is very different from a normal proprietary one but it is doable.

Having the full source code open does not make the primary author/contributor obsolete - they still have the trademarks, the needed infrastructure and the most know-how about the code. So they can much more easily do any modifications, add features and fix issues. Also the can set or very strongly influence the overall direction of the project.

So if wanted to fork the whole thing yourself, you would mostly need to invest a substantial amount of money to replicate the same capabilities the main contributor, invest more resources to keep your fork in sync with the main project and to keep any of your local modifications working. And on top of that you won't even be able to market the thing under the original known name.

So it is usually much easier (and cheaper) to cooperate in the frame of the original project - either to contribute the changes you need or pay for them to the original author, as they have the know-how to implement them for you in a reasonable time-frame. Or to pay for using the trademark, hardware adaptation, guaranteed support timeline and generally to assure that the project you are using will be there in the future.