Laziness. This is absolutely a "fuck it, we'll fix it in software" problem. I don't think it would necessarily invalidate the type certificate to change the legs but Boeing has gotten away with doing the bare minimum of structural changes necessary to keep the 737 relevant. Adding fuselage tube sections is far easier than reworking the complex gear systems and moving everything out of the way to make the bays longer (737 has an equipment bay around the nose gear, mains have systems routed around them) so they will do just about anything to avoid that.