.. because Apple has introduced what appears to be a bug whereby you can’t change refresh rates. My BenQ EX2780Q monitor has been working absolutely fine with my MacBook Pro 16″ Core i9 since I bought both earlier this year. It worked fine with Catalina, and it worked fine with version 11.01 of Big Sur (the first full public release of Big Sur).
After updating to 11.1, I was in full resolution with the external monitor, but I couldn’t set the refresh and it enabled HDR mode. Rebooted. Couldn’t get full resolution no matter what I tried. Rebooted again. Bingo. It worked, but subsequent reboots only ever bring the display down to 60Hz – way down from 144Hz. Unplugging the monitor and plugging it back in again seems to fix the issue, but this isn’t a very elegant solution.
This appears to be a bug brought forward from the 11.1 betas.
One thing I’ve found that helps – start the MacBook Pro without the monitor turned on. When the system boots into Finder, switch the monitor on and close the MacBook Pro lid. As this bug seems to randomly change resolutions, you may be in a much lower resolution than expected – but you should find that you can change the resolution and the refresh rate.
However, if you leave you MacBook Pro on and switch the monitor off to have a bit of a break, you’ll come back to find that the monitor has changed resolution and refresh rate again, possibly requiring a reboot.
You’ll also find that it takes much longer for the MBP or monitor to switch resolution versus Big Sur 11.01 or even Catalina.
This is by far one of the worst bugs I’ve encountered with macOS in over a year – and I sincerely hope that Apple fixes it soon. I’ve already filed a report with them via Apple Support and if you’re experiencing this same issue, I strongly suggest doing the same.