As to why AM's parents haven't made any media statement:
1) If they have paid any attention to news or social media over the last 3 years, they've seen all the negative attention on Kohberger's parents and sisters even though they did the "right thing" and immediately issued a well worded statement right after he was arrested and again when the plea agreement was made as well as stayed entirely out of the public eye. They made sure to acknowledge the victims', the families', and the friends/community's pain and grief, and did nothing that would detract the focus where it should be---on those groups. And they still got non stop criticism from certain factions for daring to show up at some of their son's court appearances and accusations of not doing anything to help him not become who he did.
2) I don't see why they should "have to." They don't owe the public anything, but they do owe the community and the victim's children and families the ability for those groups to be the sole focus and not detract from it. Obviously, they have been questioned by the police as to any information, and either have already been contacted or will be shortly by their son's public defender.
3) And they still might make a statement...but no one should be expected to say anything when they are still trying to process this.