Why is my MacBook Air brightness so low?

Your MacBook Air's screen tops out around 500 nits, and that's the panel's hardware ceiling, not a software cap you're missing. Every M2, M3, and M4 Air hits the same 500-nit max, so a dim screen is almost always settings, not damage. Open System Settings, click Displays, and turn off "Automatically adjust brightness" and True Tone, which warms and flattens the picture. Then check Battery and turn off Low Power Mode, which dims the screen to save charge. Last, wipe the ambient-light sensor by the camera; a smudge makes the Air read the room as dark. Unlike the MacBook Pro 14 and 16, the Air keeps no range in reserve, so that's its true maximum.