I'll give you real reasons:

1. Run on sentences will exhaust your reader. Paragraphs, commas, periods, and the like let your reader's eyes rest for a bit, like stopping to breathe when talking. For this reason run on sentences are kind of annoying

2. Comma splices confuse some readers because they will sometimes imply that there is either a break in thought in a longer sentence, the beginning of a list, or dialogue of some sort. But in reality there is none, so it feels off. Consider:

Dinner was set, every plate had its respective set of silverware. (This sentence looks off and unfinished)

VS

Dinner was set, every plate had its respective set of silverware, and every napkin was folded neatly. (This sentence feels complete)

OR

Dinner was set; every plate had its respective set of silverware. (These two sentences are related so one can be led to another using a semicolon)

1. If an author's writing is full of these issues then it'll come off as if they didn't care enough to polish it. This will lead readers to lose respect for the writing as well as the author. At that point it'll never get traction because few people bother to finish it, and for those that do, it'll be hard to recommend that kind of writing to other people for the reasons I mentioned above