Because there is such a large amount of mass in the Sun, and only a very little amount of it is lost by the fusion reactions that are occurring in the core of the Sun.

For example, the Sun produces each second about 4 x 10^33 ergs of energy. That's 400 trillion trillion watts! According to Einstein's famous equation E = mc^2, this amount of energy is equal to about 5 trillion grams lost each second. Each second, 5 trillion grams of mass are converted into energy and lost from the Sun's total mass. But the good news is that the core of the Sun contains about 240 million trillion trillion grams of hydrogen gas as available fuel. This means that the Sun can continue to loose mass the way it is now for many more billions of years because if you divide its total mass (240 million trillion trillion grams) by the rate it is converting mass to energy ( 5 trillion grams per second) you get a lifetime of about 50 million trillion seconds or several, trillion years. In fact, many changes will happen to the Sun in about 6 billion years so we never reach the point where it has lost most of its mass by the radiation it has produced.

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