You specifically mentioned React, which is what I'm responding to, but yes, using UI frameworks like JavaFX, Win32, GTK will cause your apps to look a specific way. However, while React itself may be a UI framework, in the web world, styling is done through CSS. It is not React's concern.

If you know of the UI framework that all these companies are using to achieve similar layouts, I'd be very interested in seeing what specifically that is.

Also your not wrong about browsers looking the same, but that's not out of laziness (not sure if that was implied). These days browsers are so complicated that there probably won't every be a new browser developed. It's just Chrome and Firefox, which is a little bit scary IMO.

React itself has nothing to do with theming. Also, in larger companies, design and development are usually spread across separate teams. The "lazy programmers" usually don't actually decide what the UI will look like. To be honest, your comments feel very uninformed.