Blondie
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Your observation is not entirely correct.
In reality, everything depends on the map itself (and even more on the initial save generated on the map!!!). That is, the map developer can assign loyalty to each monster! And if you have diplomacy, the percentage of joining such a monster (loyal to joining) will be much higher with expert diplomacy than without it!
This is described in more detail in the Ambera Solera textbook: The Physics of the Heroes World!
There you will find answers to all your questions.
Sincerely, VDV_forever
But on a random map, by default, even with expert diplomacy, no one joins the Necromancer, even for money... or rather, very rarely, and usually when there is nowhere else to recruit (it's a shame, of course)...
This was not the case in Heroes 3, expert diplomacy usually gave a good second army.
Maybe it's because of the Necromancer faction itself, and playing as humans, things are different?