Why do so many people assume that singer-songwriters are a 20/21st century phenomenon?

I'll give you a name S O'P - Joseph Lees, 18/19th century singer-songwriter. Creator of John O'Greenfield. By chance we know his name, but his unknown predecessors stretch back through the ages.

And BTW Spleen, some of them wrote bollocks too, but the songs didn't susrvive. Nothing new under the sun.