In basic arithmetic, multiplication is an operation with the commutative property. That means 3 * 2 has the same result as 2 * 3. However division is not commutative. That means 3/2 is not the same as 2/3. Old-fashioned words for division: dividend, divisor, quotient. Old-fashioned words for multiplication: multiplicand, multiplier, product. We have similar old-fashioned words we use with addition (which is commutative) and subtraction (not commutative). So there are 4 sets of words, all similar.
These were old-fashioned words in 1955, when I learned arithmetic. By today they are very old-fashioned.
You can ask "why" about this and many other things in a language. But languages were never "designed", so there was never a "reason for it to be this way". All that exists is history. For the history of English words, I go to the website "etymonline.com" (etymology online).
That website says "multiplicand" comes from Latin "multiplicandus" meaning "to be multiplied". So perhaps "multiplicand" dates back 2,000 years or more, to arithmetic lessons in Latin, in ancient Rome.