There is no particular reason why this happens, it just happens. It appears that macOS Big Sur has downloaded and then hangs during the installation process. One way to solve this could be to restart your Mac in Safe Mode: hold down the Shift key while restarting and then restart the installation process. If this doesn't help, try resetting NVRAM:
- Turn off your Mac.
- Hold Option + Command + P + R for about 20 seconds
- Wait until your Mac restarts and try reinstalling macOS Big Sur.
Apple also recommends resetting the SMC on laptops and desktops with the T2 security chip (you can check if your Mac has it by going to the Apple menu> About this Mac> System Report> Driver).
To reset the SMC on Mac with the T2 chip:
- Turn off your Mac.
- Press and hold Shift + Ctrl + Option for about 7 seconds (make sure the cable is plugged in)
- While holding down the keys, press and hold the power button for another 7 seconds
- Restart your computer.
To reset the SMC on a desktop computer with the T2 chip:
- Turn off your computer
- Unplug the power cord, wait about 15 seconds, and plug it back in
- Wait a few seconds and restart your computer.