Why isn't Zoho Notebook accesible through Zoho Mail??

Why isn't Zoho Notebook accesible through Zoho Mail??

There's a tab "Notes" in Zoho Mail and I expected to find my Android Notebook content there. And it seems you're working on a desktop app for Zoho Notebook. Why not simply integrate it into Zoho Mail? Or at least make it accessible via web through a link from Mail?