Good question!

I think the answer might be that those other random things you focus on that appear in your dreams are much more clearly defined concepts or archetypes.

For instance with your old friend, you have a huge assortment of experiences you associate with them, places, people. You know those things very well.

The concept of LDing of reality checks however are somewhat vague and very ill defined. There is nothing real of concrete you can associate with it, so there isn't much to work with in your dreams.

In my opinion, that's what determines the probability of success for anything you attempt in dreams. The better you know it, the better the odds it will work. A carpenter is going to be much more successful at carpentry than at brain surgery.