Studies have shown that hand-written cards or letters by individuals are comparatively more revealing of the writers' natures than messages that have been typed or sent via email or other electronic means. In his book, The Missing Ink: The Lost Art of Handwriting and Why It Still Matters, Philip Hensher bemoans the infrequency with which people write with a pen because, for one thing, the act or writing is one that helps in the development of personality. Hensher contends that people shape their handwriting first in imitation of others' and later as they form more of their own personality. Further, he remarks that handwriting is formed by aspirations towards imitating another's style, or anti-aspiration in attempts to differ from someone.
At any rate, so much of how an individual approaches the page is evident in his/her handwriting. The thank you note, then, that is written with a pen in hand leaves some of that physical touch behind.
The pen has been with us for so many millennia that it seems not just warm but almost alive, like another finger: "The moving finger writes, and having writ, moves on," Omar Khayyám writes in Edward FitzGerald's translation, and everyone knows what is meant.
Hensher points to the fact that the written message carries not just the present moment with it, but rather one's past feelings, as well, as they are incorporated in the strokes and loops unique to each person. Thus, the hand-written thank you note carries with it both message and touch.
In general, anything that is written by hand is perceived to be more personal than anything that is produced by typing. The reason for this is that a person’s handwriting is unique and personal. A piece of typed communication could really have been produced by anyone and is therefore less personal. Of course, this may be changing in our modern world because people type so many more things now with the prevalence of electronic communications.
Thank you notes should be very personal. These are notes to express gratitude for something that someone has done for you or given to you. Gratitude should be expressed in a way that is personal, not in a way that is perceived to be impersonal. Therefore, people generally perceive thank you emails to be less personal and therefore less appropriate. It is generally felt that the handwritten note is better because it shows that the writer cared enough to put his or her personal touch on the note.