The probability is 1/2 because there are only two outcomes: heads or tails. Since it is equally likely that either a heads or a tails will result from a coin flip, this means that the probability of either event is 50%.
You can compare probability to an average. Just because the average grade in a class is 80% doesn't mean that if you pick two people in the class both will have a grade of 80%.
You know from experience that if you flip a coin twice, sometimes you get tails twice in a row. That is because each time you flip the coin, the odds remain 1/2; the two flips are independent of each other. The odds of getting tails twice in a row are 1/2 * 1/2 = 1/4. So 25% of the time you'll get heads twice in a row.
The odds of flipping a coin 100 times, and getting 100 heads is 1/2^100 = 1/1.2676506 × 1030
But, if you've already flipped a coin 99 times, and gotten heads each time, then the odds of your next flip being heads is still just 50:50.
However, if you have actually flipped a coin 100 times, probably you will have gotten heads about 50 times and tails about 50 times, because this is the most likely average outcome. But, it doesn't have to be like that.