Why can't i have glasses after a cataracts operation on one eye?

You can: There is no reason why one can't wear glasses after catarcts on one eye. Usually if the cataracts are present bilaterally, the surgeon will wait until after the second eye to give the final prescription.

Yes: Depending on the visual correction needed, having a power that is very different from the other eye can cause visual problems (anisometropia). Thus waiting is best if this is the case and the sequential eye is going to be operated on soon as well.

You can: You can, but you may not need them to see clearly. This can only be determined by examination and refraction. But if you want glasses you can have them and not harm the eye, even if is as simple as you do not want to wear readers and build the reading into a pair of glasses for convenience.