Some are of opinion that copyrights would stall development and evolution of the arts. i.e. It prevents other artists from building and improving upon an existing work. The greater good of mankind would be served by doing away with this threshold.
Others want to keep copyrights to "protect" their work, which is most often an issue of pride, honour, property and the wishes of the individual artist. Reasons less holy than the anti-copyright philosophy, I'll frankly admit.

Suppose I'd draw an art in a copyrightless world, and others can improve upon it. Do they fix my flawed anatomy? Do they apply it for the greater good of mankind? No. The people most likely to make use of a copyrightless world aren't even artists. Typically my art would go through a photoshop colour filter or be redrawn and used for personal gain, whether it's "I made this" glory or commercial flyers of clashing design. Is this an improvement for the greater good?

Suppose I'd draw an art in a world with copyrights, and others aren't allowed to use it. This forces others, if they are genuine artists, to create something NEW and INNOVATIVE so that it can surpass my copyright. Although the process is more difficult for those less skilled, the leap in progress and diversity is further. Is this an improvement for the greater good?

In a copyrightless world, why would people put in the effort of creating something innovative, when they can just use what's already available? Look at the sheer amount available on Deviantart. Logic, if not inherent lazyness of mankind, would have development run around in circles, and lead to the very stall that anti-copyright would seek to overcome.

Lastly there is a practical matter of income. And although it is often used as a front to defend personal motives, it has undeniable effect in a capitalistic society. You see, if everyone can use artists' existing arts instead of pay for new arts, they would be crazy to pay. Just look at the amount available on Deviantart. Artists would no longer earn money making art and would have to give it up for a regular, paying job. With art restricted to hobbyism, artists develop only slowly, and would rarely attain the funds, time and skill for those great projects that inspire us to become artists in the first place.

My personal opinion doesn't matter. Your reasoning does.