No one is constrained to their roles. But each person has their part or parts so that everyone can focus on their role and then come together smoothly. It's teamwork.
As you can see, everyone in EXO has had the chance to step out of their immediate role at least a few times. And that's great too. Also, have you ever considered that not everyone wants to do more? I think some EXO members are content and patient.
I have fun seeing everyone do what they already know how to do well but I also have fun seeing them try something that may not be in their comfort zone. It's all good to me. It's just strange that some EXO-Ls keep bringing such things up when everyone is already doing so great. ^_^