First of all, computers can be mechanical - the first computers have been [0].
Furthermore, computers are predictable. If a CPU would not be predictable, how would you program it? If you write a program, you assume that your computer is predictable and that your instructions will be carried out. (If that assumption is not met, a fault occurred - but this happens in mechanical systems as well) In fact, you can perform computation in lockstep to detect errors.
Of course, it can be practically impossible to exactly predict a network of computers with several layers of software deployed... but that is not the issue here.
