Because the elements series are meant for typical edits and playing around, while the CS versions are for professionals. Someone using a cracked version for creating commercial products would easily lose a lawsuit, a lot of money, and their business. Simple.
So if you have $0, you get GIMP and wish it was Adobe, if you want to learn digital art you can often get Photoshop Elements bundled with a tablet (came with my Wacom Capture), if you use the software every day/week to make money you get CS and grumble-mumble about upgrade costs.