Ever notice that vocal lines played on an instrument are boring? They're often like the same few notes over and over again.
When you hear a singer doing that, it's not boring (well, sometimes it is, depending on the song). But if you were to do that on guitar? YAWN.
Maybe it's the different sounds a voice can produce (different words) that makes it not boring when it's a voice.
Imagine a song like Enter Sandman with the vocals removed and the melody played on guitar or something. The first line is only 4 notes:
(1)Say your prayers
(3)ttle one. Don't forget
(3)son, to include
The next line is those same notes again.
But it's cool when it's a voice doing it.
How do you write vocals lines vs. writing instrumental melodies?
Which do you find easier?
It's about the phrasing and the fact that there's words, that's what keeps it interesting for voice.