This is something we'll be adding. We should have the same navigation tool bar appear at the top of G2G pages for members.
Since you asked "why": It's that G2G is outside our normal software. Most of WikiTree is built on one set of code (starting with Mediawiki) and G2G is built on another set of code (Question2Answer). Our blog is built on yet another set of code (WordPress). To make the navigation bar functional in G2G and the blog, we need to enable the variables we're using on the main site.