The reason for the disparity is simple, really, but not obvious.

"Passerby" is a noun formed by combining a noun and a preposition. "Drive-by" is a noun formed by combining a verb and a preposition; the noun-meaning of "drive" does not apply here.

When pluralizing such concatenations, one generally looks at whether the non-preposition is a noun, or something else. If a noun, that noun is pluralized, and then the preposition is re-attached; if not a noun, the entire word is pluralized. Thus, as "passer" is a noun, we get "passersby"; as "drive" is a verb, we get "drive-bys". The hyphen has nothing to do with it. In fact, "passer-by" is the original form, but the hyphen was dropped in common usage.

On a side note, some hyphens really shouldn't have been dropped. It took me years to get used to "beribboned", which is "be-ribboned", but which I read as "beri-boned" when I first came across it in my youth. The double 'b' should have tipped me off (and that is why the hyphen *was* eventually dropped), but at the age of seven or eight, how was I to know? Not sure why I didn't look in a dictionary at the time, though :-\

Naturally, there are always a few exceptions, usually because the "proper" form doesn't look or sound right. Can't think of one off-hand, though. As for why "drive-bys" isn't "drive-bies", that is rooted deeply in the Germanic nature of the structure of English, and entire books have been written about this. Those who care to dig deeper will find it fascinating reading.

chaosmanor