I think in the past, aircraft engines were relatively unreliable and would often quit midair for reasons other than fuel starvation so there'd be some chance of the other engine saving you. Now, however, almost all engine failures are the result of fuel starvation and two engines will die just as quick sans fuel as one so the added complexity isn't quite worth it, from a safety perspective for piston powered aircraft. The only real advantage now is that you get to go a faster (though not 2x) in a twin usually and get to earn lots of miles on your credit card since you'll be doubling your fuel bill.
The situation on turbine powered engines is completely different. The above only applies to piston engines.
I think its because you can not go significantly faster by adding another turbine and if you got a jet plane, you probably don't care about fuel efficiency.
But as you say, you don't go significantly faster by adding another piston engine, either. Aerodynamic resistance at high speed goes as v^2, so adding twice the power increases your speed at most 70%. In practice, it will be less due to increased drag from engines in the wings, more weight, etc.
I pulled a comparison from Wikipedia:
Cessna 310 (twin): 2x240hp, max speed 220mph, range 1000mi.
Cessna 182 (single): 230hp, max speed 201mph, range 930mi.
I'm sure the twin has more useful load, but it sure doesn't go much faster or farther.
The situation on turbine powered engines is completely different. The above only applies to piston engines.