Why "oes" and not just "os"?

Does anybody have any idea why in English it doesn’t seem to be “proper” to pluralize words that end in “o” by simply adding an “s” to the end? (Examples: “tomatoes”, “potatoes”) Spanish pluralizes words that end in vowels simply by adding an “s” to the end of them. Why is that not good enough for English? (I couldn’t think of any counterexamples off the top of my head)

I think it’s a remnant of Latin.

In practice though, we would expect words ending ~os to rhyme with moss

I don’t know the actual history of that spelling pattern, but it’s consistent with English phonetics.

One potato: pǝteto
Two potatoes with an “es”: pǝtetoz

“Potatos” with just the s would be: pǝtetɑs

ETA: which is restating what Mangetout said

Why aren’t we consistent about it then? A music store sells pianos, but a farmer grows tomatoes.

It’s English. Why would you expect it to be consistent?

Piano is an Italian word. Potato and tomato came to us by way of Spanish, and were originally from Central American languages. That difference may (or may not) explain the difference.