"Why can it not be changed?"
Because Ancestry.com actively markets the user-submitted trees as research tools (knowing very well that most trees are highly erroneous and useless for *research*). To recruit new subscribers (you can't see other trees without subscribing unless you go to the clumsy and non-advertised mirror site, mundia.com).
Occasionally improvements are made that improve customers' experience, but unless you are one of the corporate owners you don't get a vote. You can subscribe, accepting the corporate policies, or not.