The problem is, these are the quantities at the start of the war.
A great war means people will die.
How many died when Changqing Immortal Sect fought against the Jiuzhou Immortal Gate?
After a battle, each faction lost more than two-thirds of its power.
Taiping Dao wanted to eliminate Changqing Immortal Sect, and the casualties were estimated to be about the same, not much less.
After the battle, out of fifteen thousand disciples, if five thousand survived, that would be considered good.
Five thousand disciples clearly are not enough to govern the lands of thirty nations. Even maintaining basic public order would be insufficient.
Therefore, we need to recruit more people, many more people.
The more prepared we are now, the greater the chances of Taiping Dao winning the decisive battle in the future.
Also, given the size of Taiping Dao, it can actually afford to support so many people.