Why is my MacBook Air at full brightness but still dim?

Your MacBook Air's panel is running at or near its real maximum, around 500 nits, which is genuinely dim in direct sun. So the screen isn't broken, and nothing was wrong with the one you almost returned. Three settings can make a maxed slider look duller than it is. Open System Settings > Displays and turn off True Tone (it warms and flattens the image) and auto-brightness, then check Battery for Low Power Mode. Wipe the ambient-light sensor by the camera too. The glossy coating also throws back reflections, so even at full brightness sun washes it out. Unlike the MacBook Pro 14"/16", the Air keeps no brightness in reserve. It's a hardware ceiling.