Most of the paintings you are likely referring to go back long before recorded human history, I would recommend posing this question to r/AskAnthropology as well.

The premise of this question hinges on the flawed notion that there exists in objective terms such things as "good" and "bad" art. Not being a scholar of art history I'm not prepared to elaborate too much further on that point.

As for why the look the way do, you have to consider the world in which Paleolithic humans would have lived. Keep in mind the simple fact that most tools and pigments used and taken for granted by modern artists did not exist at this time, that they are thousands of years old and despite being sheltered from much the elements have aged and faded over millenia and that they are painted onto solid rock which is a very different thing from paper or canvas. The purpose for many cave paintings is not fully understood and would likely give us better insight into them as well.