Yes, but email can ALSO be the same wording; and a letter (whether handwritten or typed) can be duplicated.
Especially w/ a gift-giving event with a large guest list, like a bat mitzvah.
I wrote about 100 thank-you notes for my wedding, 75 for my shower. All by hand. And believe me, the wording was the same for many, many of them. I had about 4 basic formulas, and I *did* try to make sure I wrote something different to daughters/mother groups and sister groups, so that if they compared notes, they'd see I wrote something different.
But I ran out of all that much personal to say, especially when the gift was money. (I did assign different people different parts of the sofa--"thank you for the left arm of our new sofa we're going to buy"--but otherwise, there wasn't that much personal to say.)
If I were doing it by email, I'd probably do exactly the same thing.