The reality is that Beats is also primarily a fashion company. Also with a strong brand. Also with a product line that sells at a high markup. They go together like pancakes and maple syrup. If Apple puts Beats headphones in their stores they will sell just as much as they do now. Why wouldn't Apple want a slice of that profit?
And if they make a next generation Beats headphone that can only be used with Apple products? Well, that's even more money.
Edit: asterisk: which is not to say that Apple lacks technical excellence, but the reason why they are so successful is not their engineering credentials, it's because they are able to marry aesthetics and engineering.