I'm not mad, but this is as good a time as any to drop this rant. Making prose isn't easier than making comics. It can be just a difficult, and honestly it hurts my feelings when people say otherwise.
First, the planning part is the same. Things like plotting, worldbuilding, and characterization don't change from medium to medium. Stories are still stories no matter how they're told. Second, prose may work differently from comics but that doesn't make it easier. The technical problems are different but writing good prose can be just as hard as making good comics.
For example, let's say your hero gets punched in the gut. In comics you'll have to draw the punch. That requires some knowledge of physics, anatomy, and martial arts. You've got to use your art skills to sell the idea of the punch. In prose the aim is the same, but the method differs somewhat. You'll also need some knowledge of physics, anatomy, and martial arts, but instead of llustrating it with colors and lines you have to illustrate it with just words. To have the same impact as a picture, you can't just say "he got punched" and call it a day. You've got to make the reader feel the punch, feel the shock of impact as a hardened fist drives into a man's belly with another man's weight behind the blow. There's the air that blasts out of the first man's lungs. There's the sick feeling that makes him want to curl around the pain. Suddenly he's fighting to stay on his feet. Suddenly he's trying to keep his fists up. He has to keep his fists up. His opponent has pulled back the uppercut and is angling in for another body-blow . . . you get what I mean.
If comics are concerned with how things look, then writing is concerned with how things work, and that's no easy thing. Where an illustrator can simply pull up a few images for reference a writer has to do much more research to portray the same thing with equal realism.
As to why writing is widely considered easier than making comics, there are a few reasons. Writing is built on the skills of basic literacy so it's easy to think that any literate person can do it. This is the same as saying that any person who can walk can also dance, when in fact it takes time and talent to dance beautifully. Meanwhile, illustration is a rarer skillset, with fewer reasons to develop it and more barriers to entry. Because it is rare, people assume it is more difficult when it's really more of a case of expensive equipment.
These perceived levels of difficulty affect practitioners just as much as anyone else. Writers think they have it easier, so they take it easier, and indeed it's easy to fill a page with low-effort prose. Meanwhile illustrators have a better grasp of the challenges and therefore try harder.
Then there's the ability to judge quality. Like writing well, distinguishing good prose and bad is a skill that must be cultivated. You have to read a lot of books (good books!) before you have some idea of what determines quality. On the other hand, with comics the quality can be seen at a glance. Anyone can tell, which means anyone can offer feedback. This drives up standards and feeds into the perceived difficulty.
Finally, there's also the nature of the skillsets. Writers specialize in writing. Comic-makers, meanwhile, are already cross-trained. They have to work with both words and pictures. And while it can be a challenge to master both, it's also true that the comicker can let either the art carry the writing or the writing carry the art. With writers it's all writing, all the time.