I'm sure you're aware the Russian launchers such as the
Zenit have poor weight optimization compared American launchers and especially compared to SpaceX's launchers. SpaceX couldn't match the Isp of the Russian engines so they opted to optimize weight instead.
You can't build a rocket stage from love and air alone. The 9 engines alone already weight 4200 kg. 5% of the 380 tons is 19 tons - so you have less than 15 tons left for tanks, propellant utilization and structure. The engines need 18 propellant lines with 10 cm diameter. Strong enough to survive not just the tank head pressure, but also the inevitable water hammer, when you close the valves for shut down or if you just throttle the engines.
A tiny soda can is just 0.07 mm thick at the top and 0.15 mm thick at the bottom - but keeps about the same pressure inside as a much larger Falcon 9 tank. The wall thickness of a tank capable of holding the same pressure and being made of the same material scales up linear by inside diameter: So without any special optimizations about the welds used for the tank or internal stiffeners, the tank would already need 3 mm thickness on the average - with some optimizations you can get this down to tapering between 1.5 and 2.5 mm.
The rocket itself is quite long. If you would use 2 mm thin aluminum for the tanks, a 40 meter long tube with 3.66 m diameter would already weight 5 tons. Now add anti-sloshing devices, propellant utilization, valves, the bulkheads for the tanks, stiffeners for getting the loads of the engines transfered to the second stage (or payload)... and you will be around 8 tons, and have only 7 ton left for all the rest. Thrust structure is harder to estimate, because it depends on many factors. But you can be sure that it is likely almost as heavy as the tanks themselves, because it has to transform the forces of the engines into forces that don't break rocket, especially the vibrations of the engines and the vibrations by the hydraulic TVC system have some effect there. Also the more engines you have, the heavier this structure will be. So, lets be nice and say it weights just 4 tons. 3 tons left.
Now you need to get the propellant into the engines. So, you will have 18 short pipes with 10 cm diameter, one short 30 cm pipe for the Kerosene tank and a long 30 cm pipe inside the Kerosene tank for the LOX (30 cm means the same cross section as nine 10 cm diameter pipes), include some bellows to allow the pipes to survive the vibrations... About 2 tons of mass, likely more (The pipes have to withstand some stronger changes of static pressure than the tanks). Now you have only one ton of mass free for all the electric power subsystem equipment (batteries, power controls), all the GNC electronics that you need, all the pressurant bottles and all the radio antennas, receivers, data handling, you name it. All fits barely into the 5% budget and I had been pretty optimistic in the numbers.
Now, where do you think you can reduce the mass to get below 5%? I have already calculated pretty optimistic, the second stage of the Saturn V did also just barely reach the 5% limit back then (No important electronics and much lower thrust to weight)
The second stage of the Falcon 9 could get below the 5% easily and even with very conservative calculations. But for getting even slightly below 5% for the first stage, you would need to assume perfect magic welds (Structure acts like it was made in one piece) and no safety margins too often. 5.5% (2.5 tons more dry mass) would still be a great lightweight structure, but far more realistic in its assumptions.
And in case you think about it: You can't make a thrust structure for a rocket purely of composites. The vibrations make it very hard to use composites there, only the few static parts of it allow it. Otherwise, you risk that fatigue shredders your rocket during launch - nobody has yet managed to find a way to make composites vibration-resistant and light at the same time, despite the huge demand for such a magic material.
Composites work great for interstages - but the Falcon 9 SSTO does not have many of those.
