“Why don’t people say thank you?”

Maybe I am getting to be a ‘grumpy old man?’ Some say I have been taht person for years!
I recently rana free seminar for post-graduates at Manchester Business School. Most did not have a job and I offered to send to those who wrote to me a copy of a letter they could use to apply for jobs. It is to go with their CV/ Resume and I believe will at least help them get an interview.
I received 8 requests and sent 8 letters.
I did not get one thank you back.
Now I don’t do these things to get a ‘thank you’ but common courtesy disctates when you do someone a favour, the least you can do is acknowledge it with a response.
Am I being unreasonable?