The main "problem" with the concept of DevOps is that System Administrator's #1 priority is the usability of the system they maintain. This is why we kick "high availability" and "redundancy" around a lot.
The #1 priority of a Developer is to create new functionality.
These aren't the same.... so one person can't do both completely...
You will find people who say that these aren't intrinsically disparate. This is true in an ideal world. Let me know when you find that ideal world, I want to move there.