Because it needs special support in the window manager. At that point it is the same as what Wayland is doing except it still has all the other flaws of X11 and it will break when someone wants to disable compositing or use another old window manager. As for the other parts, those would technically work, and it already does work like that for the most part. Most programs that can do DPI scaling do already have a way to scale themselves. But that is not a good experience for users, the point of it working seamlessly is to have support for it built into the window manager.
>Then i suggest making better effort towards understanding what others write or at least ask clarifications
There are no clarifications to ask, you are repeating the same comments that get posted all the time. Keep in mind these are the type of comments that you can see repeated very often:
"I have no problem, it works on my hardware"
"It is not a problem because I personally prefer it this way"
"Just try to fix the problem yourself by tweaking these developer settings (config files, environment variables, etc) and hoping it works"
If your comment follows this pattern then I would say just avoid making that comment, it is not moving the discussion forward. I think you can agree, when building these systems the goal is to get something that is usable out of the box for everyone. So these comments do not "move the needle" towards that, these are just reinforcing the current status quo.