Lots of OEM businesses start by direct selling. Eventually, some of them grow and find they are good at either selling or building. If they are good at selling, they start taking on additional products made by others and become distributors and/or dealers. If they are better at building, they often find a distributor who can handle the dealers. Or, they get big enough that they can be their own distributor and work directly with dealers.

In other words, building things and finding and looking after customers are two different skills. Few are able to do both well. Yes, at a small regional scale, direct sales to retail buyers can work, especially for simpler products. But it rarely scales. I don't know when and if new ways of connecting between manufacturers and buyers will change that in the future.