We were all DiL's once, even the oldest of us, and some of us still have ongoing relationships with our MiLs. Most of those are successful and rewarding - it is the failed ones which get all the anguished posts. I don't think we really need to ask the OP why DiLs get annoyed about what MiLs do. All that is needed is for them (some of them) to use their imagination and their memories of what it was like to have a baby or small child, and then apply the Do-as-you-would-done-by rule.
Similarly, DiLs (some of them) could just use THEIR imagination to consider just how emotional a thing it is when your baby has a baby, and how much MiLs want to be involved and helpful - and apply the Do-as-you-would-done-by rule too.
Hard to believe, perhaps, but granny has done it all before, and often got it right, too. "We are none of us infallible, not even the youngest of us." William Hepworth Thompson (27 March 1810 – 1 October 1886 - there is nothing new in the perception of the young that they are the only ones to be in the right)