The engines are not the lifting force. The wings are. Were the engines vectored straight down, the force would not lift the aircraft off the ground. The thrust, facing rearward, accellerates the aircraft at an angle horizontal to the force of gravity, and when enough energy has been put into forward motion, the wings can harness this energy in the form of lift from air moving over them. It's not a perfect comparison, but think of them as gears, much as a car with a power to weight ratio with an excess of weight can move forward.