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It's due to the nature of social interactions in India (more broadly, South Asia).

In most of our native languages, we don't directly address someone by their name, unless we've been acquainted before (sometimes even after, if it was just a formal acquaintance). We add a little something for politeness and respect. So it's always 'firstname'-ji or 'lastname'-ji, for example, in Hindi.

Since there isn't a direct carry over for this in English, people adopted sir or ma'am as replacement.

That's more or less it. It's about respect when interacting with someone else.

You'll notice that people who have been brought up in or are more familiar with western culture don't really do this.

EDIT: Unless ofcourse, your question was regarding the accent. In that case, most south asian written scripts lack certain soft tonal pronunciations of English alphabets, so most conversions result in hard sounding words, due to which you get the characteristic Indian (or again, more accurately, South Asian) accent.