Answer: It's extremely hard to build any equivalent to Fusion 360 or any full featured CAD/CAM tool. This is not a weekend hackathon problem!
There are so many fundamental building blocks, with many requiring teams of PhDs to produce: CAD kernels, constraint solvers, file format interop, sketching tools, assembly support (mating), simulation, etc... Not to mention the manufacturing specific modeling tools for sheet metal, generative design for milling & 3D printing.