Thre's many solo instruments which, on their own, play a single melody or single notes only. Clarinet, Oboe, saxophone, flute, recorder tin whistle and so on..
And we are lucky with free reed family of instruments that we can play chords also ( just as one performer)..
I write a lot of music on single line only mainly because it is not always used ( in classical context at least) as often as it could be.
The sound of single line melody is something that makes you really listen in a different way; it is ( to look at on a page)..apparently bare and unadorned ( like twigs in a winter scene perhaps)..yet it has everything possible within its seemingly straightforward appearance.
I always think presented with say a melodic line on page or screen gives so much possibility to go even more your own way of expressing the music in those symbols on the page; not forgetting the harmony has it's important place too. But with the single line on its own, it is like a pencil drawing which still carries depth and tonal value within that apparent symbolic language.
The single line melody has so much still to offer