Little mods that add custom biomes and still i had only find them once
as if they were generated only once when a chunk is created
It would be good if those mobs were more common to find

Yeah, I agree. The reason why there are so uncommon is unclear ...
Their weight in the spawner list is normal, the spawn check is the same as for animals, the spawn placement as well ...
The only reason might be that the modded biomes don't have the standard tags ...
You can see here : https://github.com/sfiomn/LegendaryCreatures/tree/1.20.1/src/generated/resources/data/legendarycreatures/tags/worldgen/biome
The tags used to make creatures spawn

ok that's fixed now with the new mob category