DIAGNOSIS Your PC's graphics settings are not detecting the monitor's 144Hz capability. This is almost always a cable or connection type limitation.
FIX You must use a specific connection method to achieve 144Hz. Connect using a USB-C to USB-C cable that supports DisplayPort Alt Mode from your PC's USB-C/Thunderbolt port directly to the monitor's USB-C port. This is the only connection on this model that supports 144Hz. Using HDMI or a USB-C cable that only carries power will limit you to 60Hz.
IF THAT DIDN'T WORK Your PC's graphics driver may be overriding the setting. Open your GPU control panel (NVIDIA Control Panel or AMD Radeon Software). Under 'Display' -> 'Change resolution', manually create a Custom Resolution. Set it to 1920x1080 with a refresh rate of 144Hz and test it.
HEADS UP The 144Hz mode requires significant power. Connect the monitor's second USB-C port to a power adapter (at least 15W) using the provided cable. Insufficient power can cause the refresh rate to drop or the screen to flicker.