It's because of the respect (formality) for elders.
In formal situations and for respected people (including all elders), plural versions are used in every context. For example, with पिता (father): मेरे पिता भारत में हैं - My father is in India. It's the same here.
Why are almost all the "grandfather" and "grandmother" sentences translated with the words दादा and दादी, not नाना and नानी? I am surprised these latter words don't come up more often, we also need to practice them.