Mobs sometimes stop just far enough away from the player to be unable to attack them

Mobs in minecraft stop when they think they have enough range to hit you, which sometimes they actually don’t. This can make any hostile mobs stop directly in front of you and just stare at you without hitting you, if you’re at a pixel perfect distance from their targetting. Most apparent with polar bears and iron golems.

If you want to try this, spawn an adult polar bear or iron golem, give yourself resistance, and just stand completely still while letting it keep hitting you. After about a minute or two, It will stop in place eventually, and will only resume when you move slightly.