Its not that Boeing wont.
Its that they cant.
Without ceding the narrowbody market pretty much entirely to Airbus for a decade.
The main component of an aircraft’s efficiency these days is the engine, it makes up the most efficiency savings when a new model is launched - hence the NEO and MAX, both with minimal changes but both with a new engine.
So Boeing decides to do a cleansheet 737 replacement - thats $20Billion committed.
Problem is, Airbus can use the same engine, or at least engine generation, on the A320 family for a development cost of only $2Billion.
The 737-replacement might be, excluding the engine, 3% or 4% more efficient than the A320, but Airbus only has to recoup 10% of the costs Boeing would have to, so Airbus sells the plane for cheaper and still takes most of the market at a profit.
If Airbus needs more, the A320 hasnt had a new wing since the 1980s, theres scope for a new wing to bring efficiency savings there as well. Now we are up to $10Billion and the aircraft are pretty much on par. Airbus still sells their aircraft cheaper and still takes the market at a profit.
Boeings problem with the 737 isnt that its an old design, its that its a design with limitations that needs to be worked around. The MCAS issue is entirely based around the issue of putting a large engine under a low wing - Boeing looked into raising the height of the undercarriage to fit the engine more conventionally but it cost too much in comparison to the A320NEO.
Airbus meanwhile still has massive development potential in the A320 design - so until Airbus pulls the trigger on a cleansheet replacement, Boeing cant either, because Boeing will lose if they do.