There's no good reason for people to use GPL either. People project all sort of idealistic stuff on this license but the reality is that healthy projects with a diverse contributor based are not at any real risk of their code being hijacked.
There are plenty of permissively licensed projects that have been around for decades. Copyleft licenses don't provide much additional protection. They impose a requirement on people that modify the software to provide those modifications under the same license. That's about it. Imposing that requirement is important to some people but not really that essential for the long term health of open source projects.
If you want to create a fork of OpenBSD kernel and call it DrEvilBSD and release it under the 100% proprietary DrEvil License 1.0, you can do that of course. The license allows you to do that. You are required to preserve the license and copyright notice, of course. For copyleft proponents, this is a wrong that needs to be corrected and they'll use big words like theft and stealing. For permissive software people this is a feature, not a bug. Do whatever you want with the software. These are both valid points of view to hold.
In practice, long lived OSS projects get more protection from the fact that they have a large amount of copyright holders (everybody that ever contributed to the code) which makes any form of re-licensing impractical. The Linux kernel will never re-license. Nor will the OpenBSD kernel. It would take the permission of many thousands/tens of thousands of developers; or their surviving relatives (quite a few are no longer alive). Not going to happen.
Anyway, the MIT license is a perfectly good license. It's widely used, well understood, easy to understand, etc. It has been around for decades. Countless of OSS projects use it. Perfectly fine choice if your goal is to facilitate others to use your software in whatever way works for them. Whether that's bundling into some proprietary firmware or distributing it in some OSS linux distribution. Giving users of the software that freedom is a fine choice.
And kind of important for operating systems. Unless your goal is to keep it out of the hands of companies that sell products to customers based on these operating systems. Which might be why there aren't many AGPL or GPLv3 licensed operating systems.
For a project like this coreutils rewrite, the MIT license makes sense. It maximizes usability across diverse systems Linux, BSD, proprietary UNIX, Windows, etc. without imposing restrictions. That’s likely an intentional choice to ensure broad adoption. There's no good reason to restrict that. Probably the developers want to see this go wherever it can go.