Yeah, I think the reasoning behind this is that it can take about a month to craft a high quality garment using the tools available and since we don't have task pausing for non building, it's unrealistic for you to be able to produce a high quality item in less than a day.
Even a very small sewing task, like sewing patches onto small holes takes about 15 minutes each and that's just about 20cm of stitching. For masterwork clothing you want the stitches to be absolutely tiny and even and perfect, and an overcoat is going to be a lot more than 20cm of stitching. And once it is done (after at least a month of stitching) you'd probably want to spend another month embroidering it and sewing beads onto it.