Seventy-eight was very close to eighty, but not when it came to the percentage of attunement with mana. Increasing that aspect grew harder with each additional point, and only certain risky methods could shorten the process.
The infusions of synthetic mana were the most approved and accepted method, but the risks weren't negligible. High fevers, rejection of the foreign substance, various illnesses, and more could spur after the act, and those dangers only deepened at the higher levels.
Moreover, the price was so high that only wealthy descendants could afford infusions, and that aspect increased alongside the level. Stronger warriors required better synthetic mana, which was incredibly expensive.
The recovery period was also something to keep in mind. Soldiers didn't only need to absorb the foreign mana for weeks or longer. Their bodies couldn't accept another infusion for quite some time, potentially forever in a few cases.