Why cant people admit they're wrong?

whenever i have an argument and i totally pin the other person, they never admit they’re wrong, even when i give them solid evidence. they always say “but you said (insert something that can be pronounced wrong but means something opposite here), and then complain about not giving a valid argument.
also, why cant people change their mind about something when arguing? if someone brings up a valid point and then i agree with that, they stare at me blankly until i give a criticism.