Here's why:
* Prefixes are word parts that are added to the beginning of a word to change its meaning. Examples include "un-", "re-", and "pre-".
* Suffixes are word parts that are added to the end of a word to change its meaning or grammatical function. Examples include "-ing", "-ly", and "-er".
The word "dessert" is a standalone word and doesn't have any parts added to the beginning or end to create it.