It may sound odd, but I think some older people, particularly, are embarrassed and feel guilty when they receive presents.
My dear late mother would always say, 'Thank you, you shouldn't have bothered'. She really didn't want me to spend money on her. I remember how hurt I was when small, saving up for a small box of chocolates for her as a surprise, which she gave straight back to me, saying, 'Here, you have them.' She didn't mean to be unkind, but I have never forgotten how I felt having my carefully planned gift rejected, and it made me even more sure to say thank you properly for any gift ever since.