It's a good question - one I've wondered myself. Thinking about it, though, the word "giants" just doesn't sound as good. I think the hard "i" doesn't scan in the rest of the song.
"Princes wait there in the world it's true
Princes yes, but wolves and giants too"
From a purely aesthetic stance, humans sounds better there than giants does.
Yes, it's a bit redundant, but as Stephanie said, princes AREN'T human in this world - they are above the rest of humanity, so don't get included in the broader sense.
Hi, Shirley Temple Pudding.