Because they are easy to find, easy to use, and are customer focused. Now, let's look at some that are considered to be average. Keep in mind that what some may consider to be good, others may consider to be bad. This return policy is not only difficult to locate, but the products are not guaranteed. Basically, you are buying at your own risk.
Returns here are possible only if it was their error.
They consider any purchases made at their store to be custom, even if you can purchase the same thing at your local computer store.