Why don't Americans write "devor" instead of "devour"?

Why don't Americans write devor instead of devour to be consistent with the pervasiveness of using variations such as color and armor?

No it wouldn't, because devour doesn't rhyme with colour/color or armour/armor. It does rhyme with hour.

In other words, the ending -our is only respelled as -or when it represents an unstressed, r-colored schwa [ɚ]. The stressed diphthongs in hour, devour, flour, etc. retain their original spelling.

English is a language influenced by Latin. The word colour/color comes from "color, -oris" whereas armour/armor comes from "armatura, -ae".

"-or"/"-our" are all Latin based noun endings that can be found in other European languages too, for instance in French or German.

"devour" on the other hand is a verb and verb endings follow different rules. "devour" comes from "devorare". The ending "-are" got dropped as centuries passed. Something similar happened to initiare -> initate, abdicare -> abdicate and many other similar words.