Bespoke CI is easy to build but no one wants to be in charge of rolling out a critical security patch to that on-prem box no one's touched since that consultant from 2 years ago.
As a really big bonus, that also makes your CI testable.
In the previous job, we built such a thing: https://smarketshq.com/building-a-reproducible-ci-system-for...
At some point, you develop complex interdependencies with other systems. You need sophisticated caching for optimum build performance. Techniques like GitOps are unsustainable at a certain number of engineers/commits per hour.