I think we can have the best of both worlds here: OS distributions that are being maintained by paid teams of security experts, and that can be audited by anybody.
What are the major ones? Android, Chromium OS, RedHat (Fedora, CentOS), and SUSE.
I think the reasons are indeed quite good, the security community or industry has a lot of black sheep.
Problem with MS and Apple is that they force intimacy by design. That itself is a security threat to me.
As for Linux, piles of companies pay for Linux kernel security, though many bugs are found by academics and unpaid independent security researchers. Linux is one of the best examples of many-eyes security. None of those brilliant and motivated researchers are allowed to look at the inner workings of MacOS or Windows though Darwin is at least partly open source so I put it way ahead of Windows here.
Also re-implementations of most features from grsecurity have been in the mainline kernel for years. https://gist.github.com/madaidan/2031b844d760683af19e85bda18...
On system-call firewalling tactics like SELinux it is true very few use these in practice as most devs have no idea what a system call is, let alone how to restrict it. That said Kernel namespacing features have come into very wide mainstream use thanks to Docker and similar containerization frameworks which cover much of the scope of things like SELinux while being much easier to use.
As for most Linux /distributions/, I sadly must agree they favor compatibility and ease of use over security basically always. I will grant that Windows/MacOS enable basic sandboxing features that, while proprietary, are likely superior to nothing at all like most Linux distros. Other choices exist though.
QubesOS is the Linux distro for those that want high security. It is what I run on all my workstations and I would trust it out of the box over anything else that exists today as hardware access and application workflows are isolated by virtual machines and the base OS is offline.
QubesOS is a Xen distro that happens to use Fedora by default as dom0 vm.
If you install something like grsecurity or use SELinux policies, I could buy the argument. I have yet to see these used in production though.
Also seL4 is not mathematically proven to be secure, it is formally verified which means it does what it says it does. Part of that spec may include exploitable code