@laurabyrne The dropdown menus follow the convention followed by the Bootstrap CSS framework. As you can see here http://getbootstrap.com/components/#navbar, the dropdown opens onclick and the parent node is always “#”. That allows touch screen users (such as those on tablets and mobile phones) to utilize the menu by tapping on the parent menu node (since there is no “hover” on touch screens).
Also, if the main node took the user away to another URL, the user may not be able to see the menu dropdown options.
Alternatively, you can use a plugin such as Mega Menu to replace the default menu.