It's a limitation of the BIOS to initialize the video card and output a supported signal.
OR you have the BIOS set to use the onboard video as primary and when Windows boots it changes it to work.
I have to disagree that 1920x1080 at 75Hz is normal. I doubt a BIOS can get that right so there's that issue.
It may be something that you are stuck with unless it's the BIOS setting I noted.
I just upgraded my monitor to ACER NITRO VG220 and I connected it to my NVIDIA GeForce GTS 450 video card via HDMI cable. When I power on my PC, the monitor keeps saying "input not supported" on a black screen until the boot sequence finishes. It shows the image from the welcome screen of windows 10 and after that it works normally. The problem is that I can't access BIOS settings or anything else at startup. I checked the resolution and the refresh rate and they are ok (what monitor specs are 1920x1080 , 75 Hz). Any idea how to make my monitor start normally?