Because evolution takes millions of generations to happen. A good example of modern evolution are the elephants, which are evolving without tusks because of poaching. I think I made my point
Read about it on Cracked.com, but the one thing I don't understand is how the elephants know that they're being poached for their ivory tusks.
The ones with tusks do not survive because of poaching. The ones without tusks and ones with faulty tusks survive and reproduce. What you're seeing is the result of elephants with generations of families that survived because they didn't have their survival and combat trait.
A bird doesn't wake up one day and say "I think I'm going to evolve wings so I can fly." Evolution doesn't work like that. The same goes for why elephants would evolve to not have tusks. It would have nothing to do with them knowing they are being poached or not. As Freakenstein pointed out, it's because those with that genetic trait to produce tusks aren't surviving to reproduce, thus not allowing that genetic trait to get passed on. Instead the ones with the genetic traits for smaller tusks get to reproduce and pass that trait on to the next generation. The result is we see elephant begin to lose tusks as a trait.
