They have invested so much emotionally (and usually financially), that admitting they were wrong would be devastating to their psyche.
I used to know someone who went so far down the hole that I doubt he'll ever come out and admit he made a mistake. He is still wearing the red cap - literally and metaphorically. The last time I saw him, he had turned to a lot of what-aboutisms about Obama and revisionist history, rather than face the truth. We used to be able to have semi-reasonable policy discussions. Not anymore. So I just do myself a favor and stay away.
I suspect that there are also some people who support Trump in these polls who know Trump is bad. But saying so would make "Libs" look good so they lie and say Trump is doing a good job. 
Edit history
Recommendations
2 members have recommended this reply (displayed in chronological order):
= new reply since forum marked as read