Because it's a shitty excuse for deliberately breaking UX.
Because it's an indication that the designer believes he/she knows better than the OS programmers, the browser programmers, and the user when it comes to site ergonomics when said belief is demonstrably misguided.
Because it's an indication that the designer believes he/she knows better than the OS programmers, the browser programmers, and the user when it comes to site ergonomics when said belief is demonstrably misguided.
Because it doesn't fix the problem.