Here's why:
* Free morphemes can stand alone as words with meaning. For example, "engage" is a free morpheme.
* Bound morphemes cannot stand alone and need to be attached to another morpheme to have meaning. In this case, "dis-" is a bound morpheme (prefix) that attaches to "engage" to create the word "disengage."
So, while "disengage" is a word, it's formed from combining a bound morpheme ("dis-") and a free morpheme ("engage").