Because the fuselage is pressurized. And it is convenient to have pressure vessels as cylinders for even distribution of of the pressure forces across it. Ideally you want to have it capped off with two hemispheres (think propane tank shaped) but obviously you can't do that for other reasons. So there is likely more design and manufacturing for the front and rear of the plane than the main section.
Ah, gotcha. Yeah, there’s zero chance that you can make a pressurized pancake using rib-and-stringer architecture, probably even out of unobtanium. The best candidate I can think of is making it like a submarine, although that would definitely mess with the open concept floor plan and probably impose a huge weight penalty (although it might be helpful from a fatigue life standpoint). The only other remotely viable solution I can think of is drop stitching, but that has some fairly obvious drawbacks that would probably make it impracticable.
Another problem is how planes are designed for different sizes.
Say you design your plane with the base / most ordered as the dash 9.
Now someone wants a smaller longer range one.
just take some fuselage barrel sections out. Boom dash 8.
Want an even smaller one. Take more sections out. Dash 7.
Need a huge massive super long range one with loads of seats.
Add sections and you get the dash 10 or 11.
Current planes use a uniform fuselage structure for this reason. You're going to make different models within the same model family with a flying wing.