Why can't people say "thank you"?

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Usually when you go to a birthday party, a gift is usually expected and the recipients say "thank you" when you hand the gift to them. Why do some recipients think it's enough to just say "thank you" when you hand the wrapped present to them but they don't take the time to write thank you notes to you after they've opened the gifts? How does the giver know if the gift was useful or not, so they know in the future? Whenever I receive a gift in person, I always say "thank you" in person. But after the gift is unwrapped, I send a "thank you" note or email to the giver and say positive things about the gift and/or that how the gift is useful. Or if the gift was tacky, I send a "thank you" note or email anyway and just be vague and say something like that it will be put to good use. I try to make use of all the gifts I receive but if I can't use them, I donate them to thrift stores so someone else can use them. I ALWAYS make a habit to write thank you notes. Plus, people can buy a pack of "thank you notes" at the dollar store or Walmart. It's really cheap. Or they can just send a "thank you email" if they don't want to spend on postage. Not receiving thank you notes, doesn't motivate givers to give gifts in the future.