Why are bicycle helmets shaped the way they are?

They have. They are called skinsuits, and all professional time trial riders use one. I think some have raised ridges sewed in, that delay air separation from the suit, thus reducing drag. Others use dimples in the fabric (same reason they put dimples in golf balls) that also delay air separation. In college, we had a lab that had a small wind tunnel and one of experiments was to test how much drag cold be reduced comparing a ping pong ball against the same ping pong ball modified with a small rubber band, and we found the band really did reduce drag even at slower air speeds.

I assume you're talking about the aero helmets, as opposed to the "normal" helmets that even pro riders wear most of the time. Normal helmets are designed to not only sufficiently protect the head, but also to be light weight and be very breathable.

Aero helmets are used primarily in time trials and by triathalon riders. In both of those cases, the peloton (The big group of bikes that travel together) is not allowed. As such, aerodynamics are very important. For example, look at a triathalon bike: http://www.trekbikes.com/us/en/bikes/triathlon/ It is designed primarily around aerodynamics.

Furthermore, for those that do use aero helmets in road racing other than time trials and triathlon racing, remember that in the peloton the head is the area that receives the least benefit of reduced drag, as such it becomes more important to be aerodynamically efficient in helmet design.