But that's the core of the problem, isn't it? People setting up devops environments are the reason these flows are so complicated. You can set up the same flows by having Jenkins call out to Jenkins to trigger chains of jobs that can conditionally fail.
People generally don't do it, because there are better alternatives out there for when you want those types of pipelines (which includes Github Actions and their counterparts). Setting up a four-step build+test+scan+tag+push+deploy pipeline in Jenkins is just as hard as it is with Github, if not harder. [Jenkinsfile](https://www.jenkins.io/doc/book/pipeline/jenkinsfile/) is just YAML with extra parentheses.
The modern YAML files are a simplification of the manual steps required to do it all in tools that predated the YAML files. There's nothing stopping you from having a Github runner configuration that's just a call to a single Docker image containing all the bash/perl/Makefile/Kotlinscript/Go/Ruby/etc. files you want to execute. Or you can forego the Docker part and run code directly inside of the repository, with all the security risks that come with it (not dissimilar to having a fully open Jenkins server).
Complicated YAML is a symptom. It's no better or worse than Bash or TOML or Makefile when you start adding conditional automation and multi-step CI/CD flows.