Why is the plural of "photo" not "photos" but "photos"

Why is the plural of "photo" not "photos" but "photos"

That's what English says
You can also understand them as follows, easy to remember them
The living, the o-ending and the plural all add es,
For example: tomatoes, potatoes, heroes
Inanimate, o-ending and plural are all S-PLUS,
For example: photos, pianos
Another understanding is the abbreviation of photogragh, so we don't use it
But just now my father said, it is mainly the habit of foreigners
There is no absolute way of understanding
But it's not all like this. It's just so easy to understand~