So why do some foods like ndizi, vaili, and yai exist as both singular and plural forms (when they are still count nouns), but maparachichi here seems to be distinctly plural?
There are different noun classes in Swahili. Some foods are in the ?/ma class (they add "ma" in the plural -so parachichi (avacado) / maparachichi (avacados), tunda (a single fruit) / matunda (fruits). Also yai (egg) is in this class, so eggs is mayai. (If you've seen "yai" translated as eggs, then that's a beta mistake. Ndizi is in the so-called "N" class (some words of which begin with n, many of which do not.) N class nouns are the same in singular and plural, although they take different agreements in each. So "ndizi yangu" is my banana, "ndizi zangu" is my bananas. The "N" class is the largest noun class, so you'll encounter it a lot.