While the 737 is a good plane, there is a reason that Airbus does better with the A320, it entered service a full 20 years later, and so its basic design is more modern.

To its credit, Boeing has been looking at a replacement for the 737, but now it’s back to square 1, as they have disbanded the 737RS (replacement study) group(paid subscription required)

The the project did not show the sufficient performance improvements to justify the capital costs of a new airframe.

They were looking at a roughly 10% reduction in costs, and felt that they needed 20-30% cost reduction, so something like incorporating Pratt’s GTF into existing airframes makes more sense for now.