Why can't I turn the brightness up on my Mac?

First, open System Settings > Displays and turn off automatic brightness. Auto-brightness, True Tone, or Low Power Mode is the usual reason the keys feel stuck in a bright cafe. If the slider's greyed out, restart, or make sure F1/F2 aren't set as standard function keys. On an external display, the keys only control the built-in panel by default. But say you're on a MacBook Pro 14"/16" or Pro Display XDR and you've maxed the slider. Then you've hit the limit macOS sets, not your display. It keeps about 1,600 nits in reserve, and MacBrightness opens that range on the same keys, $5.