I would guess because the greener pastures are still greener or otherwise maybe just as green? I moved from PHP to Ruby and Rails eight years ago. I _really_ enjoy writing Ruby (I love the syntax) and Rails servers all my web dev needs perfectly. I'm now learning about the joys of functional languages though Elixir/Erlang (a common next step for Rubyists).

The question is: so what if PHP even has caught up? What could I possibly gain from going back to it? I would have to rebuild context and learn a whole new web framework in a language whose syntax I personally do not enjoy. So no matter how good it's become, it's totally not worth my time and energy to switch back to it.