I think in childhood we all played the game "find the differences" in newspapers and magazines, while in the West they were looking for Waldo. We trained our ability to notice details. For some reason, at an older age, this experience is completely forgotten by many.
Let's be honest, most developers are unable to implement a ready-made design from ready-made mockups. And not because they lack skills. They simply don't see their mistakes. Margins, colors, fonts, not to mention line-height.
At one of our previous companies, we had a term "ass-eyes" to describe this phenomenon.
Almost every time some guy on the internet implements any mockup, I want to cry because he obviously doesn't see the difference between what needs to be done and what he did.
And same at work. Developers implementing the screen don't see it, testers don't see it, the priority of such bugs is always absolutely minimal, no one ever comes back to them.
How designers live when they see the imperfection of the world every day - I don't understand. Personally, after opening some average application or website, I can spot several issues within the first minute.
On top of this, designers often draw mockups in static, don't consider edge cases like long content, insufficient content, scrolling, animation. And developers have to figure this out in the process.
And they don't know how to do it, for some reason. Only a few can ask the right questions, not to mention making it beautiful themselves.