Here's why:
* Prefixes are word parts added to the beginning of a word to change its meaning. Examples include "un-", "re-", and "pre-".
* "Pretend" is a single word with its own meaning, and there is no separate word part that can be isolated as "pre".
The word "pretend" likely comes from the Old French word "preindre" meaning "to take, to seize", which itself may be related to the Latin word "prehendere" meaning "to grasp". So, the "pre" in "pretend" is part of the word's root and not a separate prefix.