Counting baptisms

Why, oh why do people talk about a full-time missionary’s “number of baptisms” (meaning the number of people who that missionary taught who immediately chose baptism) as if it’s a valid measure of success, or even a valid measure at all?

(And yes, i say this as someone who taught more people who were baptized than my mission’s average, so i get to ask it without it seeming like it’s sour grapes.)