I don't expect thank you letters these days. I'm happy with a text or mention on FB, and even that is more so that I know that whatever it was has survived the post. It can be worrying to send a hand-made gift (and in my experience a baby blanket will take a lot more than 12 hours to knit!) and not know if it arrived or not.
Sometimes, as others have said, the expectation of thank you letters sucks the joy out of receiving presents - particularly at times when people get a lot at once, such as weddings and when they have a baby. They are a throwback to the days when life was slower, women didn't work and had more time for things like that.
I used to get my children to ring and thank people personally, which was usually appreciated more than a letter.
To digress slightly - one thing that does annoy me is when I am asked whether my (adult) children have received a gift/card from my mother. I have no idea, as they live miles away . Also, my mum has their numbers, and can text/call them as easily as I can, and I am no longer responsible for teaching them manners.
I agree that they should thank my mum if she sends them things, but they both work in normal times, and as often as not they have to go to the sorting office to collect an item that she found in the charity shop where she works (or used to before lockdown) and they didn't really want . I have told her that they don't really want ornaments or other 'clutter', but it goes in one ear and out the other.