Codeigniter and ReactJS

I never understood this. The same seems to be happening with Laravel. As a business solution developer, in my opinion, the best flexible and maintainable approach would be:.

(1) A separate front-end (not integrated or embedded)
Where the app can be developed for mobile or Web. These can be created by using pure web stack or frameworks like React , vuejs etc These apps can interact vide API calls to the back-end
(2) A separate back-end
In most instance these would represent an API Server, headless CMS or an Admin system for the backend. These can be Python, pure PHP, Codelingiter or even Laravel (overloaded).

Using the bundled approach will lose all the flexibility of locating apps, separate maintainability, decoupling software, localization and separation of functions.  Just to my views.