The inflated HP is a huge problem with D&D based video games and was even bigger a problem in Dungeons And Dragons Online.
Enemies in that game could have thousands of hit points, but spells still followed the PnP limits of 'enemy must have under 30 / 75 hp'.
So Symbol of Death originally killed any enemies with under 75 HP, except all the basic enemies had like 2000, amd going up to 6 figures for bosses. By the time an enemy was at under 75 HP, the next standard attack swing would kill it anyway. They later reworked such spells to no longer be HP based, so symbol of death was changed to inflict negative levels instead, but that was several years after it was already in its completely unusable HP limit state.
If following PnP HP amounts, the fact is that a Wizards spells slots would last for many fights in a row as you wouldn't need to use up every spell, or such ability. In this game you pretty much need to long rest after most encounters die to how many HP the enemies have and how often your spells will still miss.
