Why exactly does private labeling a “saturated” product fail?

Title. Everyone tells me this is a recipe for failure. But why exactly? I made decent sales my first month selling a product that is very saturated. Yes, I was selling at a low price, but my sene is that even if I raise the price a bit so that I get a higher profit per unit, my product will still sell because of momentum that I picked up from historical sales. So why exactly is it a bad idea to sell a saturated product (like headphones, for example)?