I think "core" kubernetes is actually pretty easy to understand. You have the kubelet, which just cares about getting pods running, which it does by using pretty standard container tech. You bootstrap a cluster by reading the specs for the cluster control plane pods from disk, after which the kubelet will start polling the API it just started for more of the same. The control plane then takes care of scheduling more pods to the kubelets that have joined the cluster. Pods can run controllers that watch the API for other kinds of resources, but one way or another, most of those get eventually turned into Pod specs that get assigned to a kubelet to run.

Cluster networking can sometimes get pretty mind-bending, but honestly that's true of just containers on their own.

I think just that ability to schedule pods on its own requires about that level of complexity; you're not going to get a much simpler system if you try to implement things yourself. Most of the complexity in k8s comes from components layered on top of that core, but then again, once you start adding features, any custom solution will also grow more complex.

If there's one legitimate complaint when it comes to k8s complexity, it's the ad-hoc way annotations get used to control behaviour in a way that isn't discoverable or type-checked like API objects are, and you just have to be aware that they could exist and affect how things behave. A huge benefit of k8s for me is its built-in discoverability, and annotations hurt that quite a bit.