The 737 has the _fundamental_ problem of having wings very low to the ground during ground operations. Taller landing gear is not an option, in fact the 737-max has slightly-extending landing gear to resolve this issue. It is part of the reason the plane is called "max" - they maxed out things like ground clearance, fuselage length, wing chord, and other parameters.
Back to the low wings: there is no room for a large (quieter, more efficient) engine. In the earlier 737s, the engine was just as high as it could go, you could even see the cowling squished at the bottom. In the max, the larger engine is placed in front of the wing, which means that it sits much further forward than the airframe was designed for. Thus, MCAS was needed to have the plane "behave in flight like the old 737".
These are _fundamental_ issues that cannot be fixed on the current airframe. Just as the 2019 Ford Mustang chassis is better (stiffer, lighter, safer) than the 1964 Ford Mustang chassis, newer airframes resolve many of the issues that were not designed into their 1960's counterparts.
Because the plane wasn't designed that way in the 1960s, that's why. They can't change it without designing a different chassis, which means it's a different plane and a whole new type rating that pilots have to be trained for and certified on.
The 737 has the _fundamental_ problem of having wings very low to the ground during ground operations. Taller landing gear is not an option, in fact the 737-max has slightly-extending landing gear to resolve this issue. It is part of the reason the plane is called "max" - they maxed out things like ground clearance, fuselage length, wing chord, and other parameters.
Back to the low wings: there is no room for a large (quieter, more efficient) engine. In the earlier 737s, the engine was just as high as it could go, you could even see the cowling squished at the bottom. In the max, the larger engine is placed in front of the wing, which means that it sits much further forward than the airframe was designed for. Thus, MCAS was needed to have the plane "behave in flight like the old 737".
These are _fundamental_ issues that cannot be fixed on the current airframe. Just as the 2019 Ford Mustang chassis is better (stiffer, lighter, safer) than the 1964 Ford Mustang chassis, newer airframes resolve many of the issues that were not designed into their 1960's counterparts.