Why is it hard for people to comprehend that some adults don't lie?

I think it's just a trust issue. You can tell people you never lie, but you can't expect them to just blindly accept that anything you say is true until you've earned their trust, and SHOWN them that you're an honest person. So many people aren't, and it's hard to tell who you can trust and who you can't; better to be cautious than risk getting hurt.