Exactly because it's NOT "memorization" and has a specific meaning beyond simply memorizing something. I.e., you're memorizing something for a specific purpose (a performance benefit, not simply storing information).
According to Wikipedia, on memoization (emphasis mine):
The term "memoization" was coined by Donald Michie in 1968 and is derived from the Latin word "memorandum" ("to be remembered"), usually truncated as "memo" in American English, and thus carries the meaning of "turning [the results of] a function into something to be remembered". While "memoization" might be confused with "memorization" (because they are etymological cognates), "memoization" has a specialized meaning in computing.
While memorization is generically
the process of committing something to memory