なぜ，結果が character になるのだろうか

> x <- data.frame(x=1:5, y=c(3,2,5,4,3))
> sapply(x, factor)
     x   y 
[1,] "1" "3"
[2,] "2" "2"
[3,] "3" "5"
[4,] "4" "4"
[5,] "5" "3"