A friend once told me that these systems aren't designed to completely stop piracy, but to make it as annoying as possible for people to do. When you think about it, most people are not willing to try to mod a game console despite what new functionality they gain out of sheer fear of the system breaking.
As for why the PS3 hasn't been hacked, there have been compromises in security for it in the past. You all no doubt are aware of the official ability to run Linux. Well, this ability only allows some of the hardware to be accessed, not access to the more 3d aspects of the system to prevent people from making games without a license. (I'm not sure exactly what you can't access, but you can all see the point.)
In any case, I recall reading about exploits in this system that allow Linux to access the full hardware, allowing additional functionality in power. That is being "hacked" technically, it just isn't to the degree that most people prefer.
The main reason piracy related hacks haven't been enabled on the PS3, IMO, is that it simply isn't cost effective. These games can be as big as 25-50 GB, so that much download per game is simply annoying. (Granted, GTA IV release are 2 dual layer DVD's large, so this may not be as annoying for everyone, but the case with the PS3 is that every game can take up that much, not just a select few.) And Blu-Ray burners and blank media is expensive.
It's much like DVD's. When the PS2 was released in 2000, not nearly as many people had DVD burning capabilities as they do now. And it was more expensive then as well.
I'm sure that when the blu-ray burners and media prices go down like DVD's have, we'll see hacks for the PS3 to let it play copies just like the other systems.
Until then, those that wish to pirate can do so on the PC, Xbox360, and Wii. Not to start an argument, but the PS3 remains the least popular console as it is, so developers prioritize the other platforms anyway. You're not missing on much if you just pirate for the other systems.