You forgot about AC and to-hit. An ogre is designed to survive 2-3 rounds against a 2nd-level party. But his AC is so low that this requires an enormous pool of hit points. The basic problem is that, as we go up and up in level, to-hit bonuses go up, but monster ACs really don't. If you raise the ogre's AC and lower his HP appropriately, he'll still last 2-3 rounds against a 2nd-level party, and be within an assassin's chance to kill.
Basically, AC should be a larger factor in survivability so that those damage and HP curves can be brought down. Mearls stated he shot for a 50% chance to hit. In 4e, when you had 1 attack per round, the low chance to hit made the game extremely boring. In 5e, with a zillion ways to get advantage, lots of characters multiattacking, realistically, every player is hitting at least once on closer to 75% of rounds.