What you're describing is basically a passwordless setup with a poorly designed API (putting loosely defined strings that can have nowhere near enough entropy)

What you're describing is basically a passwordless setup with a poorly designed API (putting loosely defined strings that can have nowhere near enough entropy) and some great tools that have grown up around that API (password managers). When it's used well (as you do) it works great for all the reasons you describe. The problem is there's lots of room for people to misuse it. Wouldn't it be cool if we could move to a world where by design it was impossible for grandma to share her credentials from bank.com on shady-criminals-definitely-not-trying-to-steal-your-stuff.com?