I think developers can be devops also - but they are different skills you need to learn and keep up. Someone good at, say, nodejs or python data science may not be the best at CUDA build compilation on CentOS. And being good at both makes you less good at each unless you're working 18hrs a day to keep up with everything.

There is also the case of ratios. An organization probably needs more developers in specific areas than DevOps, so with dedicated DevOps you could concentrate similar work from across several teams to a dedicated DevOps team that knows that work very well.