Why is Code Igniter not recommended?

Symfony is a great framework. It is actually just a group of components so you can very easily just pick and choose what you want. It is also the fastest framework out there. The learning curve is larger, but they use the latest and best practices.

Laravel is a lot more opinionated, you really have to do it the "Laravel way". They have A LOT baked in so things like auth, messaging, queues and are ready for you to go.

CI 3.0 is dated. I would not recommend using it for new projects.

CI 4.0 is a GIANT step in the right direction. It follows newer and better practices. It doesn't have a lot baked in which gives you the flexibility to build things the way you want. I'd just study up on the 3 frameworks and see which one fits your project best.

Codeigniter is simply one of the tools you need to learn to be a successful developer. Always add more tools to your coding arsenal!