Many, many reasons... the most important of which is, Jenkins is a constant security nightmare and a maintenance headache. But also it's much harder to manage a bunch of random Jenkins servers than GHA. Authentication, authorization, access control, configuration, job execution, networking, etc. Then there's the configuration of things like env vars and secrets, environments, etc that can also scale better. I agree GHA kinda sucks as a user tool, but as a sysadmin Jenkins will suck the life out of you and sap your time and energy that can go towards more important [to the company] tasks.
I really scratch my head when I read your comment, as nothing of this is a real issue in my Jenkins.
> bunch of random Jenkins servers
Either PXE boot from an image, or k8s from an image, have a machine or pod rebooted/destroyed after one job. Update your image once a month, or have a Jenkins job to do that for you.
> Authentication, authorization, access control
Either use LDAP or Login via Github, and Matrix security plugin. Put all "Devops" group into admins, the rest into users, never touch it again.
> configuration
CASC plugin and seed for jobs, and/or Helm for just about everything else.
> env vars and secrets
Pull everything from Vault with Vault plugin.
> as a sysadmin Jenkins will suck the life out of you
I spend about 1-2 hours a week managing Jenkins itself, and the rest of the week watching the jobs or developing new ones.
Oh, good lord why?