Because the only two alternatives are 1) running mainstream distros from before 2014 or 2) running obscure distros that still don't use systemd.
Contrary to “popular misconception”, Linux is not a settler's freehold where every holdout makes their own rules.
Admins have to work with a diversity of systems. They can choose how to setup new ones, but they have to work with a range of them. What everyone does has an effect on everyone else. If some distro introduces a new way of doing things, it has some chance of ending up affecting a lot of people - they might have to adapt to also support that way, or handle it in some way.
In that way we are a village, things are connected, and that’s why there is some degree of "social control" - looking across the neighbor’s fence and meddling with their way of solving the problem - it can in some sense become ours, if we are unlucky.
Fortunately, we can relentlessly copy good solutions from others in the village too.
You can contribute to your own distro, and the "veteran UNIX admins" made Devuan. If it still counts as obscure and you don't want it to be, you - yes, you - can do something about it.
The free/open-source community is a do-ocracy. The things that are worked on are the things the people doing the work want to work on. If you have a well-paid sysadmin job where you are providing your employer value by using the work they provide you for free - and, in particular, using their ongoing work which they continuously provide you for free, because you feel like a mainstream distro from 2014 doesn't suit your needs - then you can either be grateful for what you get for free or you can contribute back.
(Which doesn't necessarily have to be contributing your own work. I'm sure if you get your employer to donate one FTE's salary to Devuan, you can change its obscurity pretty quickly!)
Traditionally developers would write code and packagers would package them into a distro specific rpm/deb -- including the init scripts (which may be pushed back upstream). Developers wanted to bypass this slow process, and there were far more developers than people willing to package the software.
Personally I've never had a problem writing an init.d script.