Although the OP is obviously meant as a mere provocation, there is an interesting aspect worth discussing:

Who is more likely to produce a good song:

• (singer-)songwriters (including teams of such, like Lennon/McCartney)
• or teams of specialized lyricists, composers, and singers?

There are many examples, good and bad, in either category, but this may be due to the specific multi-talents of the persons involved, or lack thereof.

As for old European folk songs, I guess in the overwhelming majority of cases the lyricists differ from the composers. This includes many songs that are particularly cherished for their good match of lyrics and tune. Often the gifted "matchmaker" was neither the poet nor the composer.