Why not when bonemealing manually or by dispenser on flowering azalea leaves has a chance of spawning a spore blossom? Or add it to the loot table to be the same drop chance of apples like in the oak tree loot table?

Because spore blossoms are not obtainable in vanilla survival, it doesn't fit to add them to this mod. But because they are not available in vanilla there are numerous other mods and datapacks that add them to vanilla. Install one of these mods to if you want them.