There are two things that singers have to do that a lot of pianists neglect. As a result, paying attention to these two things usually gives a huge boost to the quality of phrasing and cantabile in a pianist. When playing with a singer, attention must be payed to these two things, or the music suffers.
The two things they have to do, that pianist don't:
1. They breathe.
If a singer does not breathe along with the music, the music dies. Simple as that. A singer will correlate phrasing and register with his breathing. A lot of pianists don't do that, and as a result, the phrasing and rhythm sound unnatural, almost robotic, and they have a very difficult time following along with other musicians.
2. They work to sustain a long note.
A lot of pianists play everything staccato, not physically, but in the way they follow along with their minds. A singer can't do that, if he wants a note that lasts three or four beats, he has to be actively working those four beats and as a result, the long notes come alive and are rarely static. Pianists just press a key, and then they tend to forget about the note. Actively listening to the note, imagining a crescendo or a modulation or a change of color in the note is essential to having long flowing lines. If we don't do that, the end result might be a long note, but in our mind and interpretation we are actually playing everything staccato.