Because a nursing plan of care begins with the assessment of the admitting nurse, but carries through until the end of nursing care.
To use your first example, you have someone with a nursing diagnosis of "Imbalanced nutrition: less than body requirements." You probably made this diagnosis because your assessment, including your history-taking, disclosed a problem with eating enough or excessive caloric losses, and a history of unhealthy weight loss. What do you think some of his good long-term goals might be here? Perhaps that in 30 days his lab studies will reflect adequate protein balance (which ones would those be?), he has a better appetite and/or has ceased or decreased behaviors that burn up too many calories, and he has gained weight? You put those as reevaluation data to be measured after X time. You're not being asked to have a crystal ball, only to plan and guide someone else to do the reevaluation in X time.
How about that? Couldn't you do that on the first day you lay eyes on him, and know that they are probably correct even if you never see him again?
Of course subsequent nursing assessment may refine or even replace your nursing dx. That's OK. But it's a reasonable start, and you are empowered to make those decisions.
(PS: "malnutrition" is a medical, not nursing, diagnosis. Doesn't appear in NANDA-I 2012-2014. Think about why that is.).