Being able to say "I have deep technical knowledge of this domain, proven by the fact that I literally wrote the software and can customise it to your needs" is worth something, unsurprisingly.
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.
Architects have copyright to their work. They do sell the same plans over and over. No one can just copy their work without paying royalties or at least getting permission.
Or, in other words - the default state of things is that copyright does not exist, not that it does. It's on copyright proponents to prove that we have a better world with it than without.
Great software developers already largely make their fortune doing bespoke work for clients with a need for it. So do great lawyers, great doctors, great system administrators, great technical writers, and so on. This isn't a new idea.
Or even better, grab some other people with knowledge of the domain and go find capital so you can do what the other businesses are doing, but better, because you're backed by deep knowledge of the software that runs your company and the other companies have no clue and no money.