I think there's a bit of a 'normalization of deviance' here.

First, the kernel has a bunch of LPE (local privilege escalation) vulns. People running Linux hear about this and think "hmm, I think it's probably not a good idea to rely on user privilege separation for my security goals" and route around the problem by using other mechanisms such as VMs. As a consequence, the pressure on kernel developers to avoid having LPEs drops, because "everyone knows" that you can't really rely on user separation anyway. When LPEs get reported, they do get fixed, but with a sort of shoulder shrug. Nobody is paged at night because shared hosting is dead and everyone runs VMs.

The same thing sort of happened with containers. "Everyone knows" not to trust them so Linux namespace escapes are kind of whatever. I don't think either interface (classic UNIX users or containers) is fundamentally impossible or even that hard to secure. Virtual machines, at least the QEMU/KVM type, also expose a ton of attack surface (like a virtual floppy drive? not kidding, that has been a source of CVEs) to their guests, and they do have VM escape vulns every once in a while. It's just that we have collectively decided that their security properties are more load-bearing than those of users and containers, and we only have finite resources to invest so maybe it makes sense to at least have one boundary with okayish security?

That being said,

> Is the kernel simply too complex to reason about? Is it an underlying flaw in the security model or language implementation?

I'd say yes to both. The kernel is too complex to reason about interesting properties such as sound user separation. It's also written in a language where memory management requires very complicated non-local reasoning and even the smallest mistake often has grave consequences.