Genuine question(s):
Who pays maintainers? Are they even paid? If they aren't paid, why do they do it?
But to answer your question: Debian maintainers are not paid to maintain Debian. People volunteer to work on the Debian project because it's something they use, love, and believe in. Working on Debian is where they get their passion and joy, money comes from somewhere else.
A few maintainers few _might_ get paid by their employer to work on Debian packages that the employer has a vested interest in. As an example, there were some Canonical employees who were also Debian maintainers, but I don't know if this is still true today. I think there are a few companies that tend to hire established Debian developers, like Freexian.
Maybe if I were independently wealthy I would spend my time contributing to Debian.
Music is mostly unpaid labor and there's no shortage of musicians.
I say this as an accomplished programmer and an absolutely shit musician. the parts of programming that are fun and the parts that make money, also, have almost no overlap whatsoever
lucrative business problems that are also fun or interesting computer science problems are so rare that whole companies are built around a single interesting CS problem surrounded by a fleet of mundane business problems
This hasn't been my experience. While it's certainly true that much of my time as a corporate cog has been wasted in non-fun things like department meetings and stand-ups and Jira and MS Office documents and reviewing or debugging bad code from other devs, there's also been some time working on genuinely interesting programming problems.
But you're right about music: interesting programming assignments don't come along every day, whereas I can pick up my guitar at any time and play something (however badly) and hear something nice right away.
Many of us use Debian in our day jobs. I used to Windows at first because there was very little else, ran away to RedHat when it became clear it's very hard to develop something reliable on a base that is a black box and so unreliable and had abysmal support, ran away from RedHat to Debian at about the time of the Fedora / RedHat enterprise split because they pushed a minor update with so many incompatibilities it broke my systems.
In Debian I found a whole pile of like minded sysadmins/system builders working collaboratively on distro they can use in their day jobs. What do you need as a sysadmin - a rock solid base. Debian moves slowly, is tested for over a year prior to release, backports security patches to stable instead of moving to a new version, has hundreds if not thousands of rules and tools to enforce quality standards, and discussions with its users (who are also the developers) that span weeks if not months over technical changes to ensure they don't break things. The flips side of this coin is you will see people complaining about how old Debian is, or how hard Debian packaging is. All true. And it's that way because it's the only way we've found to create the distro these sysadmin/system builders can build on and trust. It's not a coincidence Debian lead the world into reproducible builds.
In answer to your question "why do they do it", the answer is because we haven't found a better way. Ubuntu's constant introduction home grown features like MIR, their desktop, and the snap store are designed to benefit them, but it's users. Proprietary systems are out of the question now in many places - hidden code with hidden bugs controlled by hidden entities and state actors isn't we or many of our employers can tolerate. What other way is there? It seems to me that open source (which is what Debian is - an open source distro), it the only way to build these types of systems.
My idea is to separate a 5 eur/usd from every EU/US citizen's monthly paycheck and allocate that money to the sites the user frequents, the foss tools used.