let's say for 1 player you get +10k hp, meaning for 100 player you get +1000k hp, however, all the player are not dps, you have tank and healer, them dps is far below the one of the dps. meaning a scales based on player only will become a trouble passed a certain number of player. since a lot of dps is lost by the add of tank and healer. it will recquire for the dps to deal 1.5-2 times them dps for cover the loose.

that why scaling hp are used carefully, because the situation of the fight don't often can be just judged with the number of player against the monster. other point pretty interesting to not forget it's the ilevel of the player, some will be full i170 some will be i140 maybe even below... them dps are not the same for all. simply asking to raise hp because you have more player is naive and dangerous.