A soccer field is called a pitch because the word came into sports English from cricket, where it described the playing area. Over time, British English extended it to football (soccer), and the term stuck.

Why the word changed

Originally, pitch was a verb meaning to fix or drive something firmly into place, like pitching tent stakes or stumps. In cricket, “pitching the stumps” became a common phrase, and later the noun “pitch” was used for the playing surface itself.

Why soccer uses it

When football developed in England, it was often played on cricket grounds, so the same word carried over to the football playing area. In British English, “field” more often means open land, while “pitch” became the standard sports term.

In plain terms

So the short answer is: it’s called a pitch because of British sporting language and cricket history, not because of any special shape or slope of the field.