I do tend to agree with another PP that this seems to be based on a dreamed 'ideal' grandparent that you feel your in-laws are not living up to, or even a difference in cultural or generational or straight-up personality expectation.
Although I understand having a picture in your head of what you'd ideally like for you children from their grandparents, I'm not sure it's very fair to actually expect it and get upset with your PIL if they are kind and lovely people, just in their own way. There are lots of styles of grandparenting and different styles will suit different children better or worse anyway.
My in-laws are absolutely lovely people and I adore them but my MIL in particular is an intensely private and not really demonstrative person, though I know she loves is all very much. I think she was just brought up in such a way that it would actually make her very uncomfortable to be emotionally open but she shows she loves us in all sorts of ways and is a lovely person to be around.
Also, my own mum has told me that she herself started to make a conscious effort to tell me that she loved me when I was aged 9 or so and found it intensely awkward and unnatural at first because it was so alien to her and her upbringing. However, I've always known that my mum is the one person in the world who always, completely unselfishly, has my best interests at heart and would do anything at all, regardless of the cost to her, if it helped me. She now openly adores her DGC so practice seems to have made perfect!
All of which is a very long way of saying that people are different - by temperament or upbringing - and if they are good, kind people who you know love you and your children, I think you should probably just try to focus on being grateful for that.
For what it's worth, the people who seem to get emotionally closed people to open up best are those who are secure enough in themselves to be openly demonstrative with them without expecting the same back (at least to begin with) or pushing the boundaries too much. I don't know the extent of what you're talking about but starting to encourage a closer emotional bond could be as simple as ending calls with, "ok, bye, we love you!" and not expecting them to say it back. In my experience, little things like that can really bring people up short and get them thinking about their reactions and why they don't say it themselves.
Just a thought. But it sounds like you have a lovely family, really.