Simple: there is no business model. Open Source is not a business. It is a philosophy and hobby, where people help each other and give away their labor with no expectation of a return.
(Some youngsters that have grown up in the social media age have developed a kind of entitlement complex where they focus more on the popularity of their contribution than its utility; they may be developing it just to see a lot of stars on a GitHub page. But largely it's a community driven by people who just needed some code to exist and then released it for free when it worked)
(Some businesses do release code with an open source license and even accept some contributions from the public, but largely they are doing so for various business reasons and the project is more a reflection of the business than the needs of a community. Since those projects are financed and organized by the business, they tend to end when the business abandons them; whereas a grassroots community project is often just maintained by a new stranger on the internet if the old maintainer gives it up)