The words why, wye, y sound the same but have different meanings and spellings. Why do why, wye, y sound the same even though they are completely different words?
The answer is simple: why, wye, y are homophones of the English language.
For what purpose, reason, or cause; with what intention, justification, or motive: Why is the door shut? Why do birds sing?
The reason, cause, or purpose for which: I know why you left.
Usage Problem On account of which; for which: "The reason why [regular verbs] are called regular is that we can predict what all the other three forms are” ( Randolph Quirk).
The cause or intention underlying a given action or situation: studying the whys of antisocial behavior.
The letter y.
An object shaped like a Y.
The 25th letter of the modern English alphabet.
Any of the speech sounds represented by the letter y.
The 25th in a series.
Something shaped like the letter Y.
Definitions from The American Heritage® Dictionary of the English Language, 4th Edition and Wordnik.
Homophones (literally "same sound") are usually defined as words that share the same pronunciation, regardless of how they are spelled.