Why is port forwarding NOT required for web browsers, email, or IRC chat?

when I use P2P software like shareaza I often need to manually forward certain TCP/UDP ports so that my router knows where to send traffic that is intended for my computer. When I install a new browser such and google chrome, even with my very old router, this is not required. Why not? How do these applications do it, and why can't P2P applications to use the same technique?