If we get stitches, we get marks left on the skin, but there are no marks for breast enlargements. I saw a YouTube video about a breast enlargement wherein a doctor makes a cut. Where does the cut mark go after surgery?
All cuts that go through the dermis (the full thickness of skin) will leave a scar, no matter what (and no matter what anyone tells you). The visibility of scar tissue has a lot to do with how a person heals, how much stress is put on the incision as it's healing, where the scar is, if it crosses the natural direction of skin (called Langer's lines) or goes along the lines.
Plastic surgeons have many years of training and experience in cosmetic skin surgery, and know how to minimize scar formation by their more fastidious closure methods and wound care, as well as hiding scars in ceases and along Langer's lines. But even the best plastic surgeons cannot prevent scars, only minimize them. If you still don't believe breast augmentation doesn't leave scars, you can look at this page (NSFW).
They don't always cut through the breast. The armpit is a common alternative to slicing the breast tissue. In these cases, there is absolutely no scarring on the breast of any kind.