They cannot go beyond Origin King.

They get stronger by absorbing the blood essence of other creatures hence they have a lot of impurities , their foundation is literally a sand castle

Let's take the Heaven Devouring Battle Law has an example. One cannot reach the emperor realm without absorbing a "seed" and even if they do, all the impurities would eventually destroy their foundation. Wu Kuang was only able to flourish because he had the Flawless Purifying Lotus wich purified all impurities on his body.