This appears to be an arbitrarily coded limitation in newer versions; it seems that the only way to bypass it is to use a mod to remove the limit (there is no reason why it can't be higher since it was in 1.7.10 and earlier):
Alternatively, you can give them armor to simulate more health; for example, full diamond armor with a very high amount of armor toughness (every 8 points across all pieces increases armor penetration resistance by 2 damage per armor point penetrated from a base of 2) will give them the equivalent of 5 times the health; if you also gave all pieces Protection V (or Protection 20 on one piece with +20 armor) they would have 25 times more health, then you could give them the Resistance status effect (Resistance IV = 5x more health, 125x total). Armor without any defense might be preferable since armor by itself doesn't reduce all types of damage (e.g. a 20 HP mob will die from a 23+ block fall no matter what armor they have, while Protection and Resistance will reduce fall damage). Note that some mobs do not render worn armor but do still have it equipped; you can also set the drop chances to 0 so they don't drop it.