One answer might be that it is because almost any one can write in some form or another. We all start around 5 years old with poems, "I do not like Green Eggs and Ham, I do not like them Sam I am." So many people take it the next level writing poetry, stories, papers, etc. Most colleges require some form of creative writing in their cirricleum. Even in business sources there are writing elements involved.
Quite literally anyone can do it. But writing some lines and rhymes, telling a story, is one thing. Writing lyrics, that match music, and make people physically want to purchase it with their time or their physical money is something totally different.
And with things like the internet it has turned the world into poets, composers, inventors, lyricists, etc. That is partially why the Library of Congress is backed up two years for patent applications, copyrights, and similar legal avenues.
Music usually takes some form of instruction. Whether it is piano or guitar lessons at ten years old, choir or vocal groups, or being in bands or other outlets. It takes expense of having an instrument and some form of discipline to learn to do it. And for anyone that has ever gotten that old upright piano that is used basically to sit pictures of the grandkids on or that "old guitar case underneath the bed or sitting in the corner" it is easy to lose interest fast. It is hard and takes work.
This is not to diminish great lyricists or book authors. That is extrodinarily hard to do well as well. And there are very few true lyricists. Most writers in the past 40 years have been both musicians and lyricists, which is why there are so few "outside cuts" which means the artists themselves are the writers.
Over the past years since there are so many magazines, contests, etc. that the lyrics are allowed to be submitted, the explosion of people who think they are lyricists have grown by thousands of times. But in my humble opinion, most of these are not true lyricists. One look at American Songwriter magazine's lyric winners and second, third and fourth place winners make you wonder what the criteria are. There are really no place in modern society for eight and nine verse, rambling, trite, poetry that says nothing about anything in a very poor way, outside of other rambling, trite poetry, done in a poor way writers. It frankly encourages poor writing and never goes anywhere. But it does give the winners a plaque to put on their wall. Very nice.
That is my take on the question. good to hear from you all.

MAB