While an interesting concept, I think it would be much more meaningful to actually write the letter. Make something physical of yourself and send to someone you care about. It's really fun. I love snail mail
If post cards are also your thing you could also like Postcrossing [0]! There you will sent a post card to a stranger most likely in another country and in turn you will receive one from another person. I like the idea of combining online matching and offline letter sending.
I definitely agree in many ways, but a core goal in creating this app was to meet people (including myself) where they are, which today is on our phones and computers.
Maybe delivery could be optionally executed through the mail! i.e. the writing experience happens through the app but then the recipient gets a paper letter. No idea if people would go for that or if it'd be worth the extra complexity.