Being able to pretension the cables wouldn't make them any stronger - they are still going to break under the same stress.
One of the main reasons for wanting pretensioning is so that you can control the initial sag that occurs when the simulation starts, to avoid the excessive stresses that this puts on the structure. A simple solution to this would be for the physics engine to gradually ramp up gravity for the first couple of seconds, so that it can settle into a stable position more easily. This wouldn't be totally realistic - but then bridges don't spontaneously pop into existence either.