Describe the bug
If Curios is NOT installed, mobs do not spawn with backpacks.

To Reproduce
Steps to reproduce the behavior:

1. Have Sophisticated Backpacks installed with no other mods
2. Play the game and encounter mobs, OR spawn hundreds of skeletons and zombies in creative mode
3. Notice a 100% lack of backpacks
4. Backpacks also do not drop from any of these mobs

Expected behavior
A small % of mobs spawned should visibly be wearing a backpack, and/or drop one on death.

Versions
Mod version: 3.8.1.386
Forge version: 37.0.65

Just retested this and they spawn in just fine. Easiest way to test that is raising the percentage of mobs that get backpacks in config.
Also note regarding spawning them - if you mean from spawn egg then none of them will get the backpack as in that case vanilla doesn't call the spawn event and thus the backpack addition logic doesn't run there which I don't consider a bug - the backpacks are really only meant to be added to entities that spawn naturally or from spawner.

well, I just created a completely new world with only sbp installed, upped the percentage of spawns to 50% and immediately could see them. by default it is 1% which is a chance not an exact value so if you're unlucky you can go through a few hundreds of mobs without seeing any.

And regarding eggs you're right it actually calls the event as well so the backpacks will get added in that case as well. And I have just double checked that happens without curios as well