You have an excellent point. Individual microservice containers are not complicated (then again, all they do is call a standardized script). The script will run a Dockerfile and push it to the registry. I would classify it as a 'trivial' Jenkins job, not even pipelines are used.
The pain starts when you want to do more than CI and try to get into CD. Or even worse, automate 'devops' tasks. That's where you run into all those warts.
A job could call Terraform, or spin up VMs, or run vacuum on a database, or any number of tasks. Or it may perform tasks on K8s to deploy a complex app. It may need to call APIs to figure out where to run things. And so on.
Since Jenkins is not only a CI/CD system, it can do anything, so people will try to make it do increasingly complicated stuff. And I'm arguing that this is wrong. If you have complex logic, it should be moved out of Jenkins so it can be more easily maintained and tested, and dependencies isolated. One of the easiest ways to do that is with containers. At which point, Jenkins loses most of its usefulness and other, newer tools shine.
Alternatively, use more specialized tools. If it is for CD, and Spinnaker works for you, please use that instead.