Why are news headlines in present tense if they refer to past events?

In news papers we see headings like this

"India signs a pact with Russia"

"Sachin hits another century"

"Obama wins presidential election"

These are completed events, aren't they? Then, why are these sentences not mentioned in past tense?

The headline of a newspaper was originally intended to attract the readers attention (and encourage them to purchase the paper). Framing the bold headline statements in the present tense gives them a sense of urgency and excitement that is (assumed to be) more enticing to the reader.

As other answers have said, the essence of news coverage is its immediacy. The history books will report that "the Taliban established a faux-embassy in Qatar in the middle of 2013". In the newspapers it is "Taliban open mid-east office".

If you watched a delayed coverage of a test match, would you expect the commentators to refer to each event in the past tense? Newspapers operate in the same fashion.

Even though the events are technically in the past (as is the instant when I just typed "in the past") news coverage of them is presented as though it was occurring at the same time.

This is a specific use of the verb tense known as the historical present, which means using a present tense verb to describe an event that has already happened. The excellent language podcast Lexicon Valley devoted an episode to it. They primarily discussed its use in fiction and descriptions of more remote events but their insights apply here as well: when writing a narrative of a past event, the use of the present tense gives the reader a more immediate sense of involvement. Newspaper headlines are narrative hooks to draw the reader into a story, not purely descriptive text.

News is, almost by definition, what's happening now.