Because He/She SINGS a song not Songs a song.... So Singer.
wedding singer
(from lowest to highest) baritone, tenor, alto, and finally soprano.--The lowest mail singing voice is the Bass. then Baritone, then tenor and counter tenor.In a mixed choir its Bass, tenor alto and...
Still called a band!! from wiki: Band (Music) A musical ensemble is a group of two or more musicians who perform instrumental or vocal music.
The category the singer would be in is called Soprano.