Mana in meat is like calories - it's a simple measure of energy attached to your meal, but the phydiologicalphysiological interaction with your body will tend to mess you up.
Behold the quadruple bypass burger:
One QBB will give you enough energy to:
- Last the average man nearly four days of energetic sustainancesustenance.
- Heat up one cc of water from room temperature to a plasma hotter than the sun's surface;
- Obliterate one small vehicle (9900 calories is within the range of the second row in the table).
Yet by eating a QBB you don't gain the power to go One Punch Man on a car. Instead, you get a lot of effects that include your pancreas shouting names at you, higher blood pressure, your bones getting demineralizeddemineralised from the stuff in the soda you took to down that burger and, unless you exercizeexercise harder than Michael F. Phelps, lots of extra dead weight to carry around.
Mana is just like that. You don't just store it like a battery and use it to levitate your magical bottom and shoot fireballs from your fingers. There is good mana (which is balanced on soma, ki and essence) and bad mana (which is loaded with saturated phleugmphlegm and high on vitae). If you have a very high mana intake without exercizingexercising a lot and balancing your diet, this is the kind of wizard you become:
You may have more vital energy that could in theory help you live longer, but you'll be more likely to die from a heart attack or a stroke.
And guess what... Necromancers, with their lifestyle of having the undead do their chores for them and spending a lot of time in their parents' basement doing god knows what, are usually the more sedentary mages. Even more so than those guys who teleport everywhere.
So all in all, it's not that the human eating does not do you good enough. It's just that to make full use of that energy you have to be an athlete mage, and eat other things such as newt eye and unicorn veal here and there. But healthy magical food is not as tasty, and you'd rather spend time in the magical lab than in the gym, so you don't make full use of that mana.