This is only my theory, but I think because the Chinese language is a homonym minefield that has so few possible syllables, they needed a way to distinguish nouns from other words. So, when a noun appears in its isolated form, they seem to like to add a “zi 子” as a sort of noun-making suffix. This seems to only be done to one-syllable words because, I imagine, the very presence of a second syllable (like in “píngguǒ 苹果= “apple”) gives the necessary context for people to know it’s a noun.
(it’s just a coincidence that my 3 examples are all first tone words)
But, you’ll notice when it’s put in a compound word, the “zi” drops off:
Often, if I leave off “zi 子” when I’m supposed to say it, or if I say it when I don’t need it, it doesn’t really cause a problem. But for now, my working hypothesis is: The Chinese language is one-syllable-noun-a-phobic.