I think he made his design such that he could reuse tools/skills he already had. From what he tells in the video, he seems quite experienced with welding, whereas he doesn't seem to have a good way to ensure precise dimensions or distances: There are lots of thoughts in the video how to ensure that things still work if they are wobbly.

So maybe he favored a design that let him weld components together vs a design that would require high precision when drilling the holes and making the sprocket.

Of course, when designing a commercial, mass-produced version, you'd approach this differently.