It's an arrow:
pictograms of arrows have mostly always had at least the arrow head and the shaft:
Whoever drew the cursor as we know it was drawing an arrow. It became the default standard.
You are right, it probably would work without the shaft (or tail) just as well. It's just that it's not what the UI designer chose when it was created and we've decided to stick with it.
Also, the pointer is uses as a cursor for your mouse, but it's also often needed to be shown as a tool (for example in software where the pointer is to select objects, but you use other cursors for other functions). For example, look at the toolbar on this early GUI (The Apple Lisa, which borrowed heavily from Xerox Parc's Alto):
Adding the tail makes it much more distinct as a tool icon than a plain triangle would be by itself.
DasBeasto provided an excellent link in the comments that talks about the design of the Alto Pointer:
It appears the designer was Douglas Engelbart and the angled arrow has a lot to do with the limited resolution of early GUIs. Steve Jobs and Bill Gates "borrowed" the idea and we've decided as a culture that it's simply the way mouse pointers are supposed to look forever more. :)