One minor thing to add to this: I collect old documents. (Some people spend their money on beer and whiskey. I collect old documents among other things—and probably spend less.) Sometimes we assume that documents are found in “textbook” fashion, very neat and orderly. The reality is that many are, for lack of a better word, “scribbled”. It is true that older civilizations, especially in times and places in which writing media were more difficult to come by, or in which literacy was more uncommon, were less likely to scribble. But they often did, especially in less important documents. Think of it this way: If you were taking quick notes about something, could a complete stranger, who didn’t know what subject were writing about, read what you had written? Perhaps. Perhaps not. He would have a better chance than someone who lived in a different time and was more fluent in a different language. But, in learning a foreign language, we typically learn the classic form of the letters, not the scribble.
If you have enough “classic” texts of a language, it isn’t that big of a deal. But it is just one extra factor that we don’t usually think about when we are talking about breaking the code of an unknown language.
Classic form vs scribbles? How about cursive? I learned to write Chinese characters from one of the masters of Chinese cursive. His character handwriting was totally indecipherable to me.
Also, these Harappan texts are in a durable material — it’s not unlikely that their archives were written on materials that haven’t survived. There’s a school of thought that the language of the Harappans was agglutinative, which many of the Asian languages are (along with Turkish, which originated in Central Asia), and often enough Dravidian is put forth as a likely candidate. The oldest known Dravidian texts are medieval in date and found on palm leaves, sometimes bound into books. Tamil in a written form (and archaic) is found on ostraca, including as far afield as the Egyptian shore of the Red Sea — during Roman times, a Tamil workforce was imported to make authentic pottery from India for the Roman market.