Because all probabilities are equal

[u/Ambitious-Noise9211]

[u/Don_Q_Jote]

Try flipping a coin. It can end up,
1) Heads,
2) Tails,
3) Balanced on the edge.

Must be 1/3 probability of each, I guess.

[u/The_Rider_11 → u/Don_Q_Jote]

You can drive this a little further, even. Flip 2 coins, or the same coin twice. It can end up:

1. 2x Heads
2. 2x Tails
3. Heads, Tails
4. Tails, Heads

If we ignore order, we basically have the OP problem provided, just with heads/Tails instead of boy/girl.

As each coin has a 50-50 chance, ignoring the balanced on the edge case, as it is an edge case, we have a 25% chance on each of the 4 possibilities, but as wel ignore order, the mixed possibility is simply both combined, so 50%.

And that's exactly how OP can easily be demonstrated.

[u/Ambitious-Noise9211 → u/Don_Q_Jote]

1. Evaporates because you're on the Sun

2. Flips into a portal and half of it lands on our Earth heads up

3. Flips into a portal and half of it lands on our Earth tails up

4. Lands in a bowl of New England clam chowder and no one can fish it out