Let's say you have a 6500kg piece of equipment you want to launch into space, but your rocket design can only lift 2500kg to the desired orbit. You can:
- Build five of these
- Strap them together
- Put said piece of equipment on the nose of the middle one.
That's already worse than designing a new rocket, but at least it cuts down on R&D time. The vital part is to realize that five rockets glued together are almost as powerful as five rockets launched separately (you need some extra mass to hold the rockets together and the aerodynamics are affected as well).
If you can figure out a way to transfer fuel from one rocket to another, the combined rocket has even more delta-V than the original.
Your suggestion is to:
- Build five of these
- Chop up the payload to five equally sized parts in a way that it can be assembled back together in space.
- Add an assembly machine with one of the launches, or add a sixth one
- Arrange the launches so that they meet at the same orbit. This will likely require maneuvers after reaching orbit (costs fuel).
You saved: the cost and R&D time for strapping the rockets together; the cost for a launchpad that can carry the bigger rocket (both insignificant); mass of the struts holding five rockets together (possibly less mass than the assembly drone); you lose only 1/5 of the payload, not all of it, if a rocket fails.
You introduced: the assembler cost; orbital maneuvering fuel and logistical challenge; R&D how to chop up the payload and assemble it. I'm not sure if automated docking is already a thing, but at the very least someone has to program the drone - or send a human (bad for PR if the rocket fails, and they can't handle as much acceleration).
Not a win.
Perhaps you could decide to build one rocket and reuse it five times. Unfortunately, recycling a used rocket takes time, and it still does cost money (though less than a brand new rocket). Also, it's kind of a new thing and not very thoroughly tested, meaning you might not want to rely on this. You also don't save the cost of the fuel. Overall, you might save money (or not), but I'm afraid the extra time it takes makes the idea not worth it.