The command "ng build" takes in typescript and produces optimized lazy-loaded bundles, and there won't be the need to customize that, there aren't a thousand ways to produce bundles after all, there is one optimal way for a given ecosystem, and it does not make sense for each project to reinvent a build system that all it does is transpile, concatenate, minify, compress, etc. like every other project.
Any additional build steps besides bundling are usually very simple can often be done either via npm scripts, or a simple shell script.
Can you give examples of things for which you would absolutely have to customize the build system?
The Angular CLI also supports multiple applications with shared components. You can build a web application that uses shared components, and a separate mobile application that uses the same shared components and has its own separate build.
The React and Vue ecosystems are going in the same direction with their CLIs, but I don't think the Vue CLI or create-react-app are quite as powerful yet.
Once they are, it will be the end of tooling fatigue, and no one will have to create their own custom build anymore (other than a shell script or so).