When a mob is summoned with /summon, it never drops its naturally spawned equipment, such as armor, bows, and weapons. They still drop regular items however. They have 

<Armor/Hand>DropChances: [0.0f, 0.0f, 0.0f, 0.0f], or 0 percent chance, even with Looting enchantment

Mobs spawned naturally or spawn eggs have [0.085f, 0.085f, 0.085f, 0.085f], or 8.5% chance to drop an item 

Seems to apply to all mobs.

Steps to Reproduce:

1. do /summon <mob that can spawn with armor and weapons>     Ex: zombie, skeleton, piglin
2. If it has no items, keep doing step 1 until you find a mob with both items in the hand and armor.
3. Face it, Use /data get entity <that mob> HandDropChances and/or ArmorDropChances
4. It says [0.0f,0.0f] for Hand, [0.0f,0.0f,0.0f.0.0f] for Armor.
5. Get another mob from a spawn egg or naturally spawned that has weapons and armor
6. Do steps 3 and 4 to that mob from spawn egg/natural and it says [0.085f, 0.085f, 0.085f, 0.085f]
7. Get a Looting 100 sword, looting 100 guarantees mob items to drop
8. Kill both mobs.

Expected Behavior:

Both the natural/spawnegg mob and the /summon mob should drop everything on them.

Observed Behavior:

The natural/spawnegg mob drops all its weapons and armor that it spawned with, but the one from /summon drops nothing but its usual items.