7 Answers

Possible people don't like to be told what to think and how to feel; if it is their OWN idea, that is a different story.  Some people are going to balk at changing, especially when someone proves a different viewpoint makes more sense....don't want to admit they might be "wrong".

Because you can lead a horse to water but you cannot make it drink

its like this mostly all over the world!

If it's others who are trying to change them, one would have to question their motive. If your making a conscience effort to change yourself, what's the problem? People can and do change. I don't completely buy into the "They can never change, once a dirtbag always a dirtbag" mentality. If a person is mentally stable they are capable of change. 

Adherence to already established belief systems (schemas) within themselves, which only they will change, when and if they want to.

Because they are tried and tested.

Attitudes are thinking patterns with predrawn conclusions.  This limits the amount of 'thinking through'  a person has to perform.  Its a developed pattern which will take a lot of guided work  to un-pattern.  Until then a person is actually a slave to their own making.