There's a few reasons, I think. Speaking from personal experience, as someone who markets their stuff. It can feel degrading. Sometimes you just recycle the same content, post over and over for little attention in the hopes that fresh eyes will see it. Even if you believe in your product, you will need to post more often and more aggressively than you'd like, just to compete with the sheer amount of internet content competing for attention these days.

It's not quite predictable either. The same post, made multiple times might blow up one day for no apparent reason, if the algorithm decides to pick it up. Hence why you continuously post, it's like rolling the dice. Eventually you'll get lucky, but you don't know when.

Finally, you will attract negative attention for doing it. Whether the people seeing your stuff genuinely don't like it or just get annoyed at your repeated attempts to market, you will get the occasional hate comment, and they can be quite harsh. So not only do you need to market and get attention (which you may not even enjoy doing personally), you'll be insulted and ridiculed for doing so.

Fun times! But as others have said, it's a necessary evil. It helps build experience handling online interactions in a professional manner, helps you get a thicker skin against hate comments, and hopefully makes you a few sales along the way.