> In addition, I would've guessed that the median copywriter salary would have been far lower, as my perception was that there is a far greater supply of candidates for copywriter jobs than scientist jobs in the US (maybe this is offset by fewer scientist positions?)

Probably, but given that ~20% of Americans are functionally illiterate, while ~40% have a college degree, the supplies might not be too different. Reading and writing are also skills which are neither prestigious nor especially lucrative, while scientists of any kind are esteemed pretty highly.