Here's why:
* Suffix: A suffix is a letter or group of letters added to the end of a word to change its meaning or grammatical function (e.g., -ing, -ly, -er).
* "Do-ing" is actually the present participle form of the verb "do." The "ing" ending in "do-ing" is not a suffix added to the word "do" to change its meaning; it's part of the verb's conjugation to indicate present continuous tense.
Example:
* Do: The base verb.
* Doing: The present participle form, used in phrases like "I am doing my homework."
In summary, "do-ing" is a conjugated form of the verb "do," not a word with a suffix added to it.