I am the owner of just a Series 6, but I have 5 non-OEM bands and I can say watch bands is one of the areas where Apple makes the biggest profit margins.
Some bands are made cheaper than others, most of them are just Chinese knockoffs, but I'd have to disagree based on my own personal experience. You don't have to go OEM to have nice, secure bands.
I have a bunch of Apple Watch bands both OEM and aftermarket. Probably c few dozen of all sorts starting from the Apple Watch 1 to the Ultra. For the most part, I prefer OEM, but some of the aftermarket bands are great as well. If I were to provide one criticism for aftermarket bands, it's that the attachment is sometimes loose or rattles while the OEM straps click in solidly.