Most vertical market solutions were started by advanced end users trying to solve their own day-to-day problems. Chances are they couldn't afford to hire a team of developers to build them a custom solution, so they cobbled one together themselves. Other people in their field see what they're doing, get excited, and a lightbulb goes off - "I can make a business out of this!".

Eventually it gets to the point where those solutions need to start following the rigors of a proper dev shop, but in the early days, it's more often than not the "low code" dev that you're talking about.