Ever since 0.16.0 (when the armor/tools that entities spawn with was moved to loot tables), no mobs ever spawn with enchanted armor or weapons. The only exception to this are skeletons spawned by skeleton horse traps. This is probably because enchanted items are missing from the vanilla behavior pack's /loot_tables/entities/armor_set_*.json.
First skeletons don't have enchanted bows now mobs don't have enchanted armor or enchanted weapons.
1. Start a creative world on hard.
2. Spawn 100 zombies, zombie pigman, or skeletons.
3. None have enchanted gear and enchanted weapons