Why is the “a” in “cocoa” silent?

Question

Not being a native speaker of English, one of those words that tripped me up is “cocoa”. Besides having its vowels inverted from “cacao”; it also is pronounced exactly the same as “coco”, whereas “cacao” isn't pronounced “caca” and “boa” isn't pronounced “bo”. So why is the “a” in “cocoa” silent?

Phonetic spelling from a dictionary:

cocoa |ˈkōkō|
noun
1 a chocolate powder ...

coco |ˈkōkō|
noun ( pl. -cos)
1 [usu. as adj. ] coconut : coco matting ...

cacao |kəˈkou; kəˈkāō|
noun ( pl. -os)
1 beanlike seeds from which cocoa ...

boa |ˈbōə|
noun
1 a constrictor snake ...

Answer 1

A few sources indicate it's due to initial confusion between coco, cocoa, and cocao.

According to Dictionary.com, the confusion started circa 1545 when cocao was misspelled as cocoa.

But, according to the Online Eytmology Dictionary, the pronunciation issue occurred with the 1707 printing of Johnson's Dictionary that ran coco next to cocoa, stirring confusion between the two words.