Because there is usually the head of some emperor or king or queen on the other side. To call the opposite side "tails" just seems to be as good as calling it anything else.
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tails
The probability of the coin flip being heads or tails is 100%.
Heads have a person on it. Tails have something else on it.
For a normal coin, it is 0.5.
The probability of a fair coin landing on heads or tails is even, i.e. 50/50.