For the same thrust it's more efficient to accelerate a large mass of air a small amount than t accelerate a small mass of air a large amount. The fan is what gives you that.

I rough guessed the cost of fuel over a 737's life as $150 million. Where the engines cost something around $30 million. That pushes the engineering economics towards maximizing the engines efficiency.

I'm suspicious that bypass ratio's for turbofans are close to maxed out. The diameter of the fan gets unwieldy. That was the design issue that the 737 Max was trying to get around. With bad results. Possible the future is hybrid designs with two engines and 4 or more electrically driven fans.