I can make buttons with Onclick etc. Making prefabs of the buttons and others stuff doesn’t work mostly sometimes Images sizes and rects doesn’t match buttons doesn’t work so why exactly people uses UI
And plenty of people can make games by calling graphic and OS APIs directly, without Unity or other game engine. That doesn’t mean anything
Do your buttons change the look when you hover them with mouse? Can your buttons be navigated using keyboard or gamepad? Do your buttons misclick when trying to scroll up or down using dragging motion? Do your buttons properly react when you open a popup which should prevent interacting with other UI elements? Do you have checkboxes? Do you have sliders? Do you have scrollviews? Do you have dropdown menus? Does your UI react when change in text causes some parts UI to be bigger? Do you have text input fields? Does your UI support different screen aspect ratios?
Can all of that be implemented from scratch? Yes, as long as you can read input and draw something on display you can implement almost anything. Do you want to implement all that from scratch, and how much time will it take you? A lot of people want to make games, not UI frameworks.
Not all the games need all of that. Some games get by with almost no UI at all while others are a lot more UI heavy with dozens of different menus filled with wide range of UI elements not just plain buttons.
Prefabs work just fine for me. And all the Images and rects are the exact size I ask them to be. For my projects UGUI might not be the best UI framework (compared to functionality of UI libraries aimed at making desktop software), but it’s still a lot better than implementing all that from scratch.
Yes you are right it would take lots of time . Unity is very complicated I don’t use 70 percent of the content maybe I should make my own engine in the future.