One reason for that is that websites like eBay, where you can see the individual piece of leather illustrated, blew them away. I don't think they make boots thick enough for an all-leather sheath. You can buy a sort of miniature plane with a V[shaped tooth to groove thick leather in a straight line for bending and vegetable-tanned leather will mould if briefly dampened with very hot water - not soaked and not boiling. For a ¼in. thick blade I would cut two shallow grooves with their inner edges a ¼in. apart.
I don't like the idea of stitching plus rivets. If the stitching is nearest the blade it either holds or it gets cut and lets the edge contact metal. If the rivets are nearest, it touches metal anyway. I would rather rely on very tight stitching with heavy thread, drawing the leather so tightly together than the edge doesn't reach the thread.
If you want the strongest possible thread, that is Kevlar, and second-best is nowhere. It is difficult to dye, but a pretty good match for light coloured leather, especially when waxed. But if you want a natural thread, linen is good, and resists decay far better than cotton. Whatever you use, it should be well waxed.
You can make an adequate knife from a truck spring or an extremely good one from an old file or a broken machine-hacksaw blade. But when you consider the work you will put into it, buying a piece of most kinds of tool steel is a worthwhile safeguard.