There are 2^{160} possible addresses, which is a ridiculous number. If every person on Earth makes ten addresses per second for 20 years (2x10^{18} total addresses), then the probability that at least two of these addresses collide is about 1.57x10^{-12}.

There are a lot of address, it's true. But difficulty of generating one address is the same as calculating one hash. What if somebody starts to generate address using OpenCL with speed of 4 Ghashes? This guy can get 345600000000000 of addresses every day. 63072*10

^{12} per year...

This is relatively slow, but remember - address is a limited resource. What if there are 1000000 people? What if in 10 years we can calculate 10 Thashes/sec easily?