Enantiomers have molecular structures which are mirror images of each other.
But even if a molecule has a mirror image which is different from itself, enantiomers have identical physical properties. Since most separations and purifications are done by physical processes, this makes enantiomers very hard to separate. In fact, the only way to resolve a pair of enantiomers is to react them chemically with something else which will convert them into compounds which no longer have mirror-image structures. These compounds can then be separated, and to recover the enantiomers one then has to do chemistry to break the new compounds down into their original ingredients. Actually this is not quite true.