Answer:
Because of all the random factors beyond our control that enter the flipping process (force with which the coin is flipped, motion of the air in the room, position of our hand when we catch the coin...) we therefore expect a probability of 1/2 for heads, and 1/2 for tails. Each possible outcome is equally likely.
A coin has 2 possible outcomes because it only has two sides (heads or tails). This means that the probability of landing on heads is 1/2. So, the probability of landing on heads is (1/2) x 100, which is 50%.
What he and his fellow researchers discovered (here's a PDF of their paper) is that most games of chance involving coins aren't as even as you'd think. For example, even the 50/50 coin toss really isn't 50/50 — it's closer to 51/49, biased toward whatever side was up when the coin was thrown into the air.