In South Asian culture, an unspoken norm is that you commonly refer to respected elders as "uncle" and "auntie" (in English). My understanding is that this manner of addressing elders is also present in other countries, like China and Korea.
How and when did this norm start and spread?
Unfortunately I do not have an answer, but I will add to the depth of your question by saying that similar forms of address are used in Japan.
Auntie and Uncle are used as well as Grannie and Grandpa for unrelated older male and female people whose names you don’t know. You would also refer to an older woman or man who are relatively close to your own age as older brother or older sister. If there is no more socially appropriate non gendered term of address that denotes your relationship (for example customer) you most often go for a familial term of address regardless of your actual relation to the person.