Why are enantiomers optically active?
Because they are non-superimposable mirror images.
Chiral molecules are optically active.
Enantiomers by definition, is two molecules that are mirror image to each other and that are not superimposable. This tends to apply to chiral molecules.
Chiral molecules rotate a plane-polarized light, and by definition a compound that rotates the plane of polarized light is said to be optically active .
Source: Organic Chemistry-Janice Gorzynski Smith 3rd Ed.
NOTE: If we use a pair of enantiomers in 50:50 ratio in the above picture, we will see that the light remains same (the sum of the rotations cancels out).
Being non-superimposable mirror images, they rotate the light to the same degree but in opposite directions to each other, causing external compensation, and the light appears to not have rotated.
(Not to be confused with internal compensation, which occurs with mesomeric compounds.)