I think part of this may be a hold over from horror stories of min-maxers? I remember when I was first getting into tabletop, the DM teaching me told me never tell players the HP because some players will start trying to optimize their combat instead of playing. This is just a complete guess though

My take on it as well If they know exactly who of the two same creatures has taken most damage (which happens if players take notes) they will treat it like math instead of a fight. I've tried to combat this by varying the hp with the rolled hp and using bloddied as an indicator of their health

I can't speak for other tables, but I as the GM don't reveal that information to the players not necessarily to keep it a gameplay secret but for immersion. I generally prefer to describe the state the enemy is in, how they look and the impact from the attacks than the literal hp value.