It happens to be the case that software maintenance requires essentially the same skills as software implementation, so we don’t differentiate between the two jobs. And besides that, the nature of what people demand is different: nobody expects a single-family home to suddenly accommodate 50 families, but the equivalent of this is not exactly rare in the world of software.

You forgot that maintenance contractors aren’t kept on permanent retainer. Why bother keeping software maintenance crews on permanently?

The big difference is that if a competent plumber looks at a sink, it will take them 30 seconds to figure out. If a competent programmer looks at a new codebase with 30000 lines of code, it will take them 6 months to understand it, and then still be missing 80% of the details.