A lot of people care about BSDs and embedded Linux systems. Such as: Netflix (FreeBSD), anyone with a Linux-based router or cable/DSL modem, and so on.

That said, parallel startup is possible on those systems too. There was an out-of-tree patch for parallel rc(8) on FreeBSD some time ago, and dinit is parallel, etc. I think OpenRC should move towards making parallel startup stable and supported so that those systems can do that as well.

Almost everyone's router is an embedded Busybox/Musl/Linux system. There are billions of them running every day.

Alpine has market share. It's just targeting a different market space than what you're measuring.

OpenRC doesn’t have huge consumer market share, at least in the context where this sort of thing matters. For regular Linux desktops, most of the distros use SystemD.