It's a good question, but even as a native I have found no definitive answers. Perhaps the only answer is the way Indians have traditionally perceived time. Time is a continuum that keeps going in infinite cycles, the "ahorātri" of Brahma. Yesterday determines today and tomorrow and tomorrow will define a future "yesterday" in the cycle. The word therefore remains the same, but the associated verb tense changes. Other related languages like Bengali do make a formal distinction, though. Yesterday is "gatakāl" (past kal) and tomorrow is āgāmikāl" (future kal), but even Bengali does not distinguish this in informal speech and usage.

"Kal" means "one day away from today," but you can differentiate between the two via tense. For example "Kal main khaayaa thaa" = "Yesterday I had eaten" vs. "Kal main khaaroongaa" = "Tomorrow I will eat"

Quoted from https://www.quora.com/Why-is-the-word-for-tomorrow-and-yesterday-kal-the-same-in-Hindi-Urdu See it like this: कल doesn't mean "yesterday" and "tomorrow", it actually means "one day away from today", not stating exactly when. This "one day away from today" is not used in English, so we replace it by the two words we usually use.