Why are these songs so hard to sing without backing music?

[Minnie the Minx]

What is it about these songs that makes them so hard to sing without backing music?

Nelly F - I’m Like A Bird
Kate Bush - The Man With The Child In His Eyes
Paul McCartney - Jet

Play the music and I’m away. No music, they get crucified. Why? Why just these three?

[Charlie O.]

There are loads of songs like that. I don't recall the Nelly F. tune, but the other two have some fairly unusual chord changes and modulations. The melodies don't really "make sense" without their harmonic context.

[soundchaser]

I’ve got no problem remembering music (Running Jet through my head right now), but it’s lyrics I have a problem with. Even with my own songs, I have a real problem remembering words unless they’re laid out in front of me. Everyone’s different, I guess.

[`]

[Quaco]

I sometimes test myself with weird songs to see if I can stay the course and stay in the right key. Some of the Beach Boys stuff is fun, like "Don't Talk..." and "Caroline No". The first part is OK, but it gets tricky along the way. Sounds good in the shower!

[Minnie the Minx]

EVERYTHING sounds good in the shower!

[The Slider]

I recall Tym trying to sing The Gnome once and his flailing around trying to find the right note for "and then one day..hooray!" was a highlight of my trip to Barcelona
Unusual modulation, as Charlie says.