"Scarborough Fair" is a ballad abt impossible love-tasks, that in the middle lists a bunch of herbs.

BUT!

It didnt always!

other versions:
Sober & grave grows merry in time
Every rose grows merry w time
There's never a rose grows fairer w time
Yesterday holds memories in time

it's very likely that the "parsley sage rosemary and thyme" list is a long-developed mondegreen of a mishearing of "grave grows merry in time" ("parsley" is the last-attested herb, and wasn't canonical until Simon & Garfunkel came along)

the idea of grief becoming merry would fit with the song's other concerns with the performance of impossible faerieland love tasks

i kind of like that the song is filling up with herbs over the centuries - like the song itself has gone to seed, and become overgrown

"sober graves grow merry in time" - like a song, that should be about a love that can never be, sprouting herbs.

listen to one of the ways it used to sound:

anyway, give a song 800 years and strange things start growing in it.