Because it's not a question and it's not a business related issue. It's just a small hook to get you to respond to her. There is no reason for you to do it. You're too busy enjoying your life and doing your things to respond to something like that. She wanted to send you good wishes and you go them. If she wanted to have a conversation about something, she can do that instead of using this approach - not that she wants to, but I am just saying.
You're not ignoring her, but using smart communication. She walked out and doesn't want to do anything with you and the marriage, but then sends a message like that. It could be guilt, regret, genuine caring, who knows. It doesn't matter. You're on your path.
I don't think you replying something like 'Thanks for the message' is going to hurt your chances for anything in the long run. It's more about doing less for her and being NC/dark so you can handle your business. These messages invariably make the LBS think about everything and puts them in a bad mental spot when they're trying to move forward.
Again, it's not a big deal if you respond, I just think it's better if you don't.
