External monitors often too bright with MacBooks?

Solution 1:

The range of brightness of a monitor is dependent on what the manufacturer designed and is irrelevant to the computer (Macs and PCs) attached to them.

Lumens or Units of Visible Light

The amount of lumens a display puts out (how dim and bright it goes) is totally dependent on how the manufacturer designed and built it. For simplicity, think of it like the "volume" control for audio, but instead of sound, it handles light. The fact that it doesn't go to "zero" (or low enough for your particular needs) is due to the design - there may be a technical reason like the LEDs that the manufacturer used may only work properly within a certain voltage range.

Computer Control

Your MacBook (or any laptop) can send control signals to dim or brighten the monitor but when it reaches it's lowest point, that's as far as it goes. Your computer can't make it go any lower.

Finding Your Monitor

Now, to find a monitor that suits your needs, it's unfortunately going to be "trial and error." Go into a store and test them out. If online, you'll likely have to test out buy and return a few (could get expensive because it's technically not faulty so return shipping would be on you). Hardware Recommendations here on StackExchange may be helpful, but you'll need to be much more specific as to how dim you want this monitor to be because hardware manufacturers rarely (if ever) tell you how low their monitors can go - it's usually max brightness they promote.