Thats because the "real" product they are selling
is the data collection. Also, subscription services are much more attractive to companies because its a guaranteed and predictable revenue stream.
Also, doing everything over the internet to centralized "cloud" servers is easier to develop for and manage, and its easier on the end user to set up, strengthening the product. Although I think this is more of a cherry on top of the big pile of data they're getting on you.
It also probably subsidizes the unit cost. Companies would much rather sell you a "cheaper" device with a subscription plan than a potentially more expensive device with one-time payment. Your average consumer will choose the device that is cheaper up-front for the same reasons.
The technical, privacy-focused crowd that tends to populate this forum is already being served by the type of cameras you can already buy on Monoprice, Newegg, or some other related store. Can you imagine a company like Amazon selling that kind of product? I sure can't.
With that said, if anyone would sell a privacy-focused device with all the same benefits as Ring, it would probably be Apple since privacy is part of their brand. I don't think the odds of that happening soon are high, but I could be wrong.