Why is center field deeper than left or right?

In baseball, why is the distance from home plate to the center field wall longer than the distance down the lines? Though there have been plenty of wacky outfield configurations in baseball parks over the past hundred years or so, I haven’t encountered a single case where this seemingly sacred custom has been transgressed. Does anyone know when and why such a convention got started?

Square fields were easier to find than fan-shaped ones in
the early days.

Now it’s the rule: