Here's why:
* Morphemes are the smallest units of meaning in a language. They can be individual words (like "cat" or "run") or smaller units that can't stand alone (like prefixes, suffixes, and roots).
* Morphemes combine to form words. For example, the word "unbreakable" has three morphemes: "un-" (meaning not), "break" (meaning to break), and "-able" (meaning capable of being).
* Each morpheme contributes to the overall meaning of the word. Removing a morpheme from a word often changes or eliminates its meaning.
While letters are the building blocks of words, they don't necessarily carry meaning on their own. A word like "cat" can be spelled differently ("kat" or "katt") without changing its meaning. However, removing the "c" or "t" would dramatically alter the meaning.
Therefore, the morpheme is the essential element of a word that directly contributes to its meaning.