A couple of reasons. First, a typed letter is much more legible than my handwriting. I do sign all of my letters which can give you an idea of how bad my handwriting is. It would be better if I take some effort to improve, but this takes much longer. If you have poor handwriting, I ma sure you can understand.
Second, my typed letters are smaller than what my handwriting can be. By typing the letter, I can put more content in a page. This means a 4 sheet handwritten letter can be reduced to 2 sheets – therefore reducing the cost of mailing the letter, and the cost of resources consumed.
I had a couple of people that stopped writing to me because all of my letters are typed. If this will be you, then you should consider not writing me. I will just have to deal with the loss of a shallow penpal.