Learn English – Why do some words end with -or and others with -er or -ar

Wikipedia gives a pretty good overview of the rules. Based on the information there (don't cite it in a paper, but it's good enough for our purposes), nouns ending in -o are accompanied by nouns ending in a sibilant sound, which is far beyond the scope of this answer. But at any rate, here's the relevant sections from the Wilipedia article on English plurals:

Where a singular noun ends in a sibilant sound —/s/, /z/, /ʃ/, /ʒ/, /tʃ/ or /dʒ/— the plural is formed by adding /ɨz/. The spelling adds -es, or -s if the singular already ends in -e:

kiss kisses /ˈkɪsɨz/
phase phases /ˈfeɪzɨz/
dish dishes /ˈdɪʃɨz/

...

With nouns ending in o preceded by a consonant, the plural in many cases is spelled by adding -es (pronounced /z/):

hero heroes
potato potatoes
volcano volcanoes or volcanos

However many nouns of foreign origin, including almost all Italian loanwords, add only -s:

canto cantos
hetero heteros
photo photos

So in summary, add -es if:

• the word ends in -(consonant)o (usually), unless it isn't actually an English word and we're just borrowing it.
• the word ends in a certain kind of consonant sound, simplified to most uses of "s", "z", "ts", "sh", or "ch" (plus, I think, a couple others, but those cover most cases).

There are several reasons, three of which stand out:

1. If you keep the original spelling, connections with the same word used in other languages remain intact. In honest and psychology, for example, this makes it easier for people to communicate across English, French, Latin, Greek, etc. In German, they spell Zentrum. This makes it harder for us to remember how it is spelled, and sometimes this even makes it harder to learn the word for learners. Similarly, Dutch spells akkoord, which is again less consistent with many other languages and the language of origin, French.

2. It is terribly hard to get people to change the way they have always spelled words. Imagine if you had to change your spelling all of a sudden after having spelled a certain word the same way for 30 years! Or, what may be even worse, regular spelling changes occurring every 5 years, which nobody will be able to learn or keep up with, as in Dutch. And, even if you decide that something needs to change, there will be 1001 opinions as to how. Should it be onest? onnest? onnist? As it is, no spelling system in the world is wholly phonetic, not even new systems. Imagine you had to read a teks laik this, weh ai'uhv tsyowsin suhtin konvensyuhns.

3. How will you take into the account the fact that people in different regions pronounce the same word differently? Should all spelling be based on the phonetics of Received Pronunciation? Or should each region use its own spelling, making, say, British harder to read for Australians and vice versa?