Conversely, they're not locked up tight such that there's only One True Browser etc, which kinda matters to power users.
Power users doing exactly what with their browser?
Installing a different one, e.g. because they want a better UX, or some features that the standard one is missing.
Firefox, Edge, Chrome, Opera, and more are available on iOS. Each have their own unique features and UX. The only thing not allowed is a rendering engine other than WebKit via the iOS webview component.
Spyware? Any evidence of this or you are just high on Apple Kool aid / Google hate?