I believe that originally it was more a function of college students spending their free time on it. Of course, this results in addressing fewer enterprise-level use cases.
The individual incentive is to open source code to show the rest of the industry what you have been able to build, with personal returns to you if it actually gets meaningful adoption.
And companies are willing to go along with this because infrastructure projects are not actually a source of competitive advantage and they would rather co-operate on keeping input costs low, and also not get stranded on a tech island with totally custom tooling for everything. You also derive some marginal benefit if the OSS code actually gets used by others since you still control the project direction.
And then separately you have OSS first companies that are hoping that being OSS lets them gain a marketing/sales advantage with developers who care about that.
These incentives remain the same even if there is no meaningful surplus.
Now, just as food for thought: I believe the mere existence of FOSS promotes a more sustainable economy!
It regulates, in a way, commercial software. You cannot be more user hostile / be less convenient and/or provide lesser of a service than FOSS alternatives! Convenience is at the end of the day, the reason for someone to choose a tool. Software is a tool. Greater convenience of use generally promotes greater productivity when using the tool.
Many of us already make the logical decision to use FOSS to not have to deal with the rising user hostility of consumer grade software. Maybe because, we as developers, have the know how to minimize the rough edges or have the same type of brain of the creator of the software in question, and think: "Indeed! the decisions made on the interface, the flow of using the tool, whatever else, are sane and make the most sense!!!". Hinting here, that the incentives of FOSS do not necessarily align with the preferences of a consumer, which is sometimes a good thing.
Nevertheless! The freedom, as in free, not as in beer, represents a bar of sympathy, of respect for the user. One that has the potential to minimize user hostility of commercial software for fear of losing that customer. Users are beginning to be more savvy than ever, they've been burned a lot before, and/or were born in the midst of the digital age. The barrier of FOSS, its arguable lack of convenience, is shrinking every day. And, in such a way promotes an economy where you have to respect your customer. One that is then, more resilient and sustainable.
It's originally the product of amateurs, in the original sense of the word: people doing something for the love of it.