TalonsUpPuckDown
Roll along you BG Warriors!
Even on airplanes, fewer engines is better....down to a point. If you're flying over water, redundancy is nice. But fewer bigger engines is such a huge cost incentive that the regulating agencies have developed a certification called "ETOPS," (which officially stands for "extended twin operations" but is better known by running industry joke as "Engines Turn Or People Swim"). Without ETOPS, twin engine aircraft have to be within 60 minutes of flight time (on a single engine) of a suitable divert runway at all times. You can get ETOPS ratings to allow longer times, up to 330 minutes, which basically covers 100% of the earth's surface.
So you can be a less nervous flyer, ETOPS certification is based not only on the design characteristics of the airplane ("how many redundant fuel pumps do you have?") but also on the actual service history of the planes, as maintained by the operator. So if United maintains their planes better and experiences fewer failures as a result, they may be able to get certified for more minutes of ETOPs than American can for the exact same type of aircraft. Going for an ETOPs cert therefore means that the airline has committed to collecting and documenting all the right failure rate data (in perpetuity, not just a one-time thing) to prove to the FAA that ETOPs is low risk - and going to all that trouble is still cheaper than adding a 3rd or 4th engine.
Fun fact: The 777 was the first aircraft to be ETOPS certified at the time of first customer delivery. https://www.amazon.com/Twenty-First-Century-Jet-Making-Marketing-Boeing/dp/0684807211