Hostile mobs will not spawn naturally (mostly) in the Nether. Endermen will spawn, and so will blazes but not unless you're near a blaze spawner. There are no piglins, hoglins, or striders to be seen anywhere.

I've had this problem with several Minecraft Java versions in the past, but I was able to resolve this issue by restarting the server, increasing view distance and/or spawn rate, or as a last resort, starting a new server. I've gone through countless tutorials and suggestions online but nothing has worked for 1.16.2.