Why do you think this is? And are these polls as inaccurate as those that promised a Clinton victory last November?
Theory 1: The GOP Base and Strong Democrats Are Immovable Objects
One way to explain Trump’s stable approval is to simply look at who does and doesn’t approve of him and how strongly they hold those views. The situation is simple: Trump has a high approval rating among the GOP base and low approval ratings among the Democratic base, and unless there are truly exceptional circumstances, neither group will change their mind.
And BTW, these data aren't from just one poll, but from the RealClearPolitics average of 9 different ones, including right-biased ones such as Rasmussen Reports and Fox News and left-leaning ones such as ABC News/Washington Post and Bloomberg.