Because long ago it was "she singeth" (I don't know why) and "she sings" sounds like that.
lala sings it prima j sings homegirls/
who sings that thing you do???
Urbandub sings it.
who sings bacon
Rey Mysterio sings it.
Avril Lavigne sings this
noone sings that
Joe sings himself.
Twista sings wetter
Ryan miller sings it
Usher sings omg
Jesus sings to the sky