Hypothesis: compound nouns that are unhyphenated single words can be pluralized by adding an “s” to the noun root only when they consist of a noun + preposition.
This is a follow-up to an earlier question
Numerous examples are given in the Answers to that Question listing compound nouns that pluralize in the middle by adding an “s” to the head or root noun, such as sons-in-law and secretaries general.
However, other than passersby, (the word posed in the question and the word that got me thinking, before I found that question), and standersby (for which I cannot find an online definition), all of the examples in that Q&A, and those that I can think of or find, are either hyphenated or written as an unhyphenated noun phrase.
Virtually all of these spaced or hyphenated examples are either noun + adjective or noun + prepositional phrase. The only exception in the examples from the earlier Question is spoonsful, but this seems to be a less favored variant of spoonfuls (although Microsoft considers spoonfuls incorrect and spoonsful correct).
There are other single word, unhyphenated compound constructions that serve as nouns which consist of a verb form + preposition which pluralize the usual way, such as
Is it possible that the reason for the passersby type construction (plural noun root, no space, no hyphen) is the particular combination of noun + preposition? This appears to be a rather rare construction.
I have reviewed word lists in onelook.com for any construction with a [word]+s+preposition by searching *sby and other *s[preposition] combinations. (I could not search *s+in because of the volume.)
The only other examples I have found are strikesthrough and sneaksby. The only reference to strikesthrough is a Wiktionary entry which considers the overall word to be a verb, but several of the listed cites appear to be typos. Sneaksby is actually a singular form.
One or two examples are a poor way to make a rule, but could it be that this particular plural form is limited to this particular parts-of-speech structure?