No respectable company sells rooted Android phones. Several factors contribute to companies' refusal to sell rooted devices. These include the inherent user protections and ability to maintain consistent brand images that limiting root access provides.
Under normal circumstances, apps run on unrooted phones operate within their own sandbox, which is essentially like giving each app its own account with its own set of restrictions. However, providing root access to an app allows it to operate outside its own sandbox so that it can obtain information from other apps or access parts of the Android ecosystem that it wouldn't normally be allowed into.
Allowing apps to run outside the operating system's typical sandboxing protections also exposes the system to problems caused by unknowledgeable users. A user might, for example, be able to delete critical system files on a rooted phone and yield the device totally unusable and unrepairable.
Manufacturers and service providers ensure that customers don't delete their built-in apps, and Google prevents users from blocking ads by restricting root access.