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.