I don't know. I can see where you're coming from but it's clearly annoying the OP so there may be merit in drawing a clear line in the sand rather than being passively accepting and being irritated in private.
Calmly confronting the issue after all this time may shock them a bit. 'I wanted to ask, why don't you use my name when you're addressing post to me? To be honest, it makes me feel uncomfortable and like you don't respect me as a person.' It deals with the in-laws' passive-aggressive campaign and clears the air. They might say 'well, we don't accept that you didn't change your name, etc etc etc' to which they only response is 'well, I'm sorry you feel that way but this is what DH and I have agreed and it works for us. Would you mind using my real name in future?'
Flogging, I've clearly said that I have not and would not take someone to task over the appelation on an envelope, unless there was something broader behind it (like in the OP's case). I don't like the assumption, but I shrug most of the time. The only time it really happens is at Christmas when people are sending joint cards, it's never happened on a birthday card.
If it's an out-of-date assumption, then as I say, it's not a big deal. But if it's an intentional, passive-agressive statement that the sender disagrees with your choice, then it does matter because it's no longer just about the envelope, is it?
It does irritate me, though, if people have seen my name written down a number of times and they still default to my husband's name.
It's worth noting that a lot of people, young and old, do make the effort to find out what people like to be called. I always remember one of DH's friends privately thanking me for checking what her baby daughter's last name was before our wedding rather than assuming she had her father's last name - it took a minute on my part but made a big difference to her.