Because it is stealing. No matter how you want to rationalize it, you do not have the right to take the property of another (physical or intellectual) without restitution. If you feel the price is unfair, you have the option of not buying it. This isn't a life sustaining product we're talking about, so don't use that "oh but Sony/Microsoft/whoever is so rich " argument.
Now have I pirated stuff? Absolutely. But, I'm not going to pretend it was in any way justified.