Rather than ask why manga writers can't give American characters a "plausible name", maybe ask yourself why you find names that aren't of Anglo-Saxon derivation implausible?
(My reaction isn't entirely to this comment; I recently saw a bunch of people in the comments on Flying Witch complain about an American character with the surname "Flores". It's worth noting that two of the ten most common American surnames are Garcia and Rodriguez.)
Our exchange student here — the transliteration is changed by ch480 to Parry Copelin — is presumably named in a double reference: to Commodore Matthew Perry, whose forced opening of Japan in 1854 led to the Meiji Restoration, and to Ichigo Marshmallow character Ana Coppola, the little English girl who can't speak English.