It's deceptively complex. What looks like buttons and sliders ends up having a lot of interactivity that is underestimated. Then there is the complexity of both

It's deceptively complex. What looks like buttons and sliders ends up having a lot of interactivity that is underestimated. Then there is the complexity of both the software and hardware stack, from different displays and devices to a rapidly evolving ecosystem of libraries, frameworks, and such. Add to this the complexity of the networking stack (at least for web apps) and it's not hard to see why even UIs with modest functionality become very large and complex. There is also something to be said about how organization structure their teams to build UIs and APIs, which is to scale productivity by adding and leveraging more people, as oppose to leveraging more powerful code. This is probably more true on the frontend as it is typically not as critical to the business as the backend, so it is not surprising to see UIs constantly break or lack any proper standardization.