Why do my breast implants have symmastia?

They shouldn't : Cleavage can't be created if you are not born with it. Implants will make your breasts larger, but they should not cross the midline(symmmastia). This can be repaired. Please search out a board certified plastic surgeon with a significant amount of breast experience.

Synmastia: Synmastia implies that the two breasts are blended into one and occurs as a complication of breast surgery not done well. It is a rare complication and usually requires the expertise of a Plastic surgeon to correct the problem

Synmastia~"Uniboob": Synmastia is generally, but not always, caused by excessive dissection in the midline (sternal area, breastbone) in an attempt to create cleavage . It may occur with either over or under the muscle placement of implants. The best treatment is prevention. Once it occurs, it can be difficult to correct and may require use of other implantable materials such as acellular dermal matrix or other option