The same reason they call you by your last name when you are talking to Customer Service., they are trying to be respectful.
Most Indians are spamming your Inbox with outsourcing requests. They blast out the same message to thousands on the mailing list. Without knowing your name, saying, "Sir" for the first meeting is likely to get them a better response.
It does piss me off though to be addressed by my last name without the Mr. infront of it. This leads to some fun Customer Service conversations. It really makes my day when I am at someone else's house and they keep calling me by the person's last name, even after I keep correcting them saying I'm not that person.
In the end, though, it comes down to them following their company's script. Nothing more.