I think you're assuming that most software work is confection.
In reality, or at least in my case, pretty much all the money I've ever earned was in doing bespoke work.
You _could_ see contributions to FLOSS as loss leaders; though that wouldn't be accurate, since there are definitely benefits beyond just advertising your skills.
A key benefit: if there is a set of freelancers working around a single FLOSS code-base, each of them actually benefits by contributing back; because the shared code-base increases in quantity and quality, and thus leads to competitive advantage for all.
