Simply, aircraft engines are designed to be able to produce a lot more than their "fair share" of the thrust needed for flight.

In the case of a twin engine aircraft, if one engine fails the other has to be able to compensate so the flight can continue safely, especially at takeoff when the plane is at its heaviest. Twin engine aircraft can easily fly with one engine out but the additional drag imposed by the ferried engine can reduce the ability for the flight to continue under the power of one engine.

Ahh, wasn't thinking of the drag aspect. Thanks!

No problem!

There are other factors too, like increase adverse yaw which would become more pronounced if the ferried engine were on the same side as the failed engine.