DIAGNOSIS Windows is defaulting to a basic display driver or you're using the wrong cable/port. The monitor can do 144Hz, but you have to set it up correctly.
FIX Right-click your desktop, go to Display Settings > Advanced Display Settings > Display Adapter Properties for Display 1. Click the "Monitor" tab. In the "Screen refresh rate" dropdown, 144Hz should appear. If it doesn't, you're likely on HDMI. This monitor's HDMI port is version 1.4, which can't do 144Hz at 1080p. You need to switch to the DisplayPort or Dual-link DVI cable that came in the box. Reboot after swapping cables.
IF THAT DIDN'T WORK Download the latest drivers for your specific graphics card (NVIDIA or AMD), not the monitor. After installing, open the NVIDIA Control Panel or AMD Radeon Settings. The refresh rate option in there is more reliable than Windows. Create a custom resolution if you have to, but 1920x1080 at 144Hz is a standard mode it should pick up.
HEADS UP Using a cheap or old HDMI cable, or even a single-link DVI cable, will physically lock you at 60Hz. The included cables are certified for the job—just use those.