Mostly because Adobe has a very large market that relies on their proprietary product, so they are able to charge users an arm and a leg for the software that they "need".
Originally Posted by drunken_dev
For creating web graphics what would be the best choice?
Adobe's photoshop is pretty much the best solution for graphics editing, as far as functionality and usability go: but, if you can't afford it(you certainly wouldn't be alone), then the best choice for you might be a comparable, free solution like GIMP. Furthermore, if you work as part of a team, and everyone uses photoshop, then the best choice would be photoshop so you can integrate with the team better.
Photoshop is the good tool, especially when it comes to designing websites and for logo and graphics, specifically for large print job, illustrator is the best software as it generates vector based graphics so even when you make them large they don't get blurred unlike graphics made through photoshop. second to illustrator is Coreldraw.