Well, that is a great question. I believe that as technology advances and even the youngest generations are constantly using electronics to communicate. This letter writing and thank you notes are kind as well as necessary. Pretty soon, the technology will take over every aspect of our lives, if it hasn't started to already. Just saw a question from a woman who said that 7 schools are completely eliminating basics of education, allowing students to use electronics to find answers, solve problems and even write.
Aside from all that, writing a thank you note is significant but not required. It is significant because written notes tend to stay with one for a long time and they can look at it years from now and remember the good times. Writing letters and sending them also keeps the Post Office running and it gives forgotten joy of receiving something from someone that matters rather then bills bills bills and spam all the time.