This might be a stupid question but why preg_replace_callback puts every match in a single-element array and passes it to the callback function so I have to use $matches[0] to process a match, why doesn't it pass matches to the callback function as strings?
Because you can have capturing groups, and each element in the array would be what the capturing group captured. The element at index 0 is always the entire match.
For example, given a regex that matches MM/DD/YYYY dates, you might put each segment of the date in its own capturing group, perhaps something naive like this:
(\d{2})/(\d{2})/(\d{4})
Then, you'd have a matches array similar to:
[0] - MM/DD/YYYY
[1] - MM
[2] - DD
[3] - YYYY