Why aren't there any single turbofan airliner? A configuration which looks like a DC-10, without engines mounted under the wings. Let's say 737 sized.

Let's forget about redundancy security advantages of twin or quad engines, since they're obvious and not my concern about this particular question.

One single engine is more efficient than two similar engines. Efficiency multiplies, two engines are less efficient, four even worse.

Center of gravity problem doesn't exist since wings can be moved backward.
(of course yaw stability will be corrected inherently)

Modern turbofans family can provide a single stock powerful enough version to fly a 737 class size.

In the event that debris is released from the engine, one canard configuration may be designed, in order to prevent hydraulic failure and loss of control.

To summarize, are there (except redundancy safety concerns) any reason why there are no single turbofan airliners?
Airliner designs must be certified in order to fly commercial services. In the US, this means compliance with the Code of Federal Regulations, Title 14, Subchapter C, Part 25. In §25.901(c) it says:


  For each powerplant and auxiliary power unit installation, it must
  be established that no single failure or malfunction or probable
  combination of failures will jeopardize the safe operation of the
  airplane except that the failure of structural elements need not be
  considered if the probability of such failure is extremely remote.


Engines do fail from time to time, so there must be a second unit which can take over if one fails. It's the same for fuel pumps, instruments or even wheels: Every one has a second unit which can take over if the first fails. For wheels, this is very easy to check: You will not find a single airliner landing gear leg which has only one wheel. All have two wheels or more.

Or pilots: The minimum number of pilots for safe operation is two. One can fly the plane (so she/he can land it if the other pilot is incapacitated), but two are needed to start any revenue flight.

I could continue that list, but I guess the concept has become clear: For every element on an airliner there needs to be a second, redundant one, except for structural elements with a very remote probability of failure.

This has been learned the hard way: Some of the first aircraft designed for passenger service (like the Junkers F-13 or the Fokker F.II) had a single engine. Some of the multi-engined ones even became dangerous death traps when one engine failed. Over time, the desire to increase safety made redundancy the supreme design principle for airliners.

Military operators have fewer qualms about single engine designs, because using fewer engines improves performance and lowers acquisition and operation cost.