The simple answer is: "because there's no easy way to install it".
The first two generations of PV+ logic modules had a "VersaView CE" version that allowed you access to the CE desktop, and included VNC server and viewer pre-installed.
The ordinary PanelView Plus terminals don't allow access to the CE desktop (the desktop components might not even be part of the CE build) and don't have VNC Server or Viewer installed.
When the PV+ 6 hardware and Version 6.0 firmware came out, they had a specific part number for "extended desktop features" that gave you access to features like VNC.
I would be happy with a way to get an old PV+ terminal to just run Windows CE and Remote Desktop Client or VNC Client, so I could point them to a remote desktop server running some other more powerful HMI. But I don't know of a way to do it.