Why is the plural form of piano "pianos" and not "pianoes"?

Plural of nouns ending in -o:

Nouns ending in -o can add either -s or -es in the plural, and some can be spelled either way.

As a general rule, most nouns ending in -o add -s to make the plural:

So you have:

solo solos, zero zeros, avocado avocados

Note that:

Those which have a vowel before the final -o always just add -s:

like:

studio studios, zoo zoos, embryo embryos.

So:

piano, pianos

Oxford Dictionary