Yabbut… discrete LEDs have their own substrate (likely ceramic), electrical pads on the bottom to make contact, and even with a center pad for better thermal contact, there’s still some insulation going on.
With COBs, they tend to be put on the common substrate, and I think are almost directly connected to the Al plate underneath (hence the “chip on board” moniker), all common-ground, leads connected directly to the chips, so while the plate still might not be the best as far as radiating heat away, they do make better contact with the cheapie chips themselves.
There might be a few or even dozens of different ways to make COBs, but I think (someone correct me if mistaken) that’s the allure of COB systems, a more direct, simplified way of sticking LEDs onto a base. Think of it as an XHP-From-Hell vs discrete XM-Ls.