It's expensive for two reasons.
- Piracy.
- Greed.
They would charge a higher price than it is truly worth because they know their clients will fork over the cash in order to use the best tools on the market. This is their own greed. To make matters worse, they know piracy exists on their products and therefore as a reactionary response they make the price low enough that their clients will stay pay but high enough that they aren't putting themselves into the hole with piracy / lower than assumed sales.
Gimp is nice, but Photoshop is easily better. And if you're really serious about something, wouldn't you want to use the best tools on the market?