[Jtsummers]

Because white goes first. The phrasing and context becomes important. There are two statements that are true but without them as context you end up with a misleading notion:

First player wins more than second in chess (because there's a slight advantage).

White goes first in chess.

Therefore, white wins more than black in chess.

However, that conclusion (what you present) is valueless without the context. If you reversed which color starts the game then we'd end up with "black wins more than white in Chess" as a conclusion and your statement would be false. And if there was no specific color which always started, we'd be left with just the first statement: First player wins more than second in chess.

[BeefWellington]

First player advantage seems to be consensus but I wonder how much of an effect piece layout has for players playing second.

If you reversed the king and queen placement for both players would the advantage be as great?

[InitialLastName]

Unless there are hidden psychological effects (which good players will attempt to surmount, as a matter of course), mirroring the board placement will make no difference.

There is no asymmetry of moves other than the starting position of the king and queen, so all strategy will simply be mirrored if the king and queen are mirrored.