Because instead of helping me enter a long filename by its distinctive parts, it forces flat mental read-decide-skip loop based on first few letters, which is slow and cannot be narrowed down without typing out additional letters by hand. It is like selecting from a popup menu through a 1-line window.

If it presented a real popup for file selection, combining both worlds (tab to longest match, untab to undo, up-down to navigate), that would be great. But it doesn’t.