I've just done a three-page song which has only four expression marks for the pianist in it, and only one for the singer. There's an fp at the beginning, a couple of mfs and a crescendo mark for the pianist, and a crescendo mark for the singer. I suppose you could count a few odd staccatos and emphases marks as well, but they're fairly minor and hardly to be worried about.
Some songs I've done (I only transcribe songs for other people) have almost as many marks as this per bar, which makes the work of transcribing very tedious. It also means that the composer doesn't trust the singer and pianist to work out how to perform the song satisfactorily for themselves. Most experienced musicians and singers have a feel for how things should go, and don't need all this additional instruction. Most of it will be done instinctively, so for the composer to write it all out is just a bit pointless. Better for him or her to stick to the vital things, the changes of expression that aren't obvious from the music itself.
It's a bit like a scriptwriter telling the actors in every line how they should say the words. Actors will ignore these things, unless, again, it's something that you wouldn't expect from the words themselves. Shakespeare never writes such stage directions, and people manage perfectly well to interpret his lines. The very few directions he does write are exits and entrances, and the occasional curious one that you'd never gauge from the rest of the script. In fact a lot of his stage directions are written into the dialogue, as it were, and can be worked out from there.
Compositions are similar to scripts for plays, I feel. While there are purists that claim to know how every note should be played, it's really up to the performer in the end, and the majority of composers will give performers plenty of leeway on this. A very few don't seem to feel that performers have the first idea about how music should be 'done', and clutter up the page with more instructions than any performer will pay attention to in a lifetime of performing the work.