They aren't bad. In the short term they can even be the better solution. However you're dependant on one company to develop the tools you're using. This can work ok but you don't have much influence over the direction those tools go. Also you are helpless if the company goes bankrupt, is bought by another company or simply discontinues the product (or service).

If crimes are so bad, why do people keep committing them?

Because they learned to use those programs in art school, because art school chose to teach those programs, because companies ask for them in the job market, because companies think these programs are taught in art schools, and the cycle repeats.

IT IS CALLED VENDOR LOCK-IN!

That's individual for every user of the particular software. I'm using free software to get my work done faster.

Hi! I do like propietary SW, however open source SW usually is more helpful, in the long term talking.

If cars do harm to the environment, why are people still using them?

Proprietary programs are not bad, they are restrictive about your freedom: if you want more freedom about your artwork, you should use free softwares. Most people don't know any alternative, and some people despise free software because they aren't commercial. So most people still use them... help us spread the world about libre graphics!