Lots of reasons for this, here are a couple: Having extra power helps you climb to cruising altitude quickly. This is a significant consideration, as a great d

Lots of reasons for this, here are a couple: Having extra power helps you climb to cruising altitude quickly. This is a significant consideration, as a great deal of fuel expenditure takes place during the climb through dense, drag-inducing atmosphere. Engines and aircraft structure are non linear in both up-front cost, maintenance, and economy. Two larger engines have about half as many parts as four smaller ones, impacting production costs and service requirements. Larger fans are also more efficient than smaller ones for a variety of reasons. Fuel systems and aircraft structures are also simpler, lighter, and less expensive with fewer engines.