The reason is rooted in how browsers work.

In native software you can "just" render your data structures directly to the screen. In the browser, barring webgl, you can't render your data structures directly to the screen, instead, you're forced to interact with an imperative browser API to individually manage bespoke UI widgets that store their own state internally and can't understand nor access any of the data structures defined for your business logic. The only option left is to manually update the internal state of the browser UI widgets whenever you update the internal state of your application. Manually keeping the two states in snyc is a real pain, highly bug prone, and a maintenance nightmare. The front-end browser frameworks came along as a way to "auto-sync" the business logic state with the browser ui state.