The goal of GeneratePress is to be lightweight, fast with features that are useful for the majority of users.
Page builder features are generally heavy for the sites and a lot of our users choose not to use it – especially with the new possibilities introduced by the new Gutenberg editor.
Therefore page builder functionalities are definitely in the plugin territory so that the people who want to use it have the option to do so when they wish. We do make sure GP is compatible with all page builders 🙂
Hope this answers your question.