Too many competitions
Although selling on marketplaces guarantees traffic, it doesn't guarantee the priority and visibility of your products. When customers search for a product, both you and your rival's product will appear. Then, your customers will compare which product is cheaper and more reliable. Hence, you're in a price war with your rivals.
Besides that, for each sale you get, the marketplace takes in a certain percentage of your profit. Instead of dividing your profit, you can open an online store to attract customers to your store alone and keep all the profit to yourself. Sounds like a good plan?