It is not that they are incapable of creating a brand new item in any color or enchant they want. They've proven numerous times over that they already have the ability to spawn anything form mobs to items.
The policy is because investigating the truth of the claim requires a lot more work. They would need to store backup saves of your account data in order to be able to "roll you back" as it where and restore stuff. From what I can figure, Nexon/devcat is conserving resources by limiting the amount of data that needs to be saved per user. I imagine there is only one "save file" for each of your characters. Pets are separate characters, so when one is desummoned, it gets saved in it's own file. Your character has it's own save file. When you experience the crash... there is only one saved data to go to for you when you log back on... the one that didn't have the item you pulled off it.
An extra backup per character would double the amount of date needed to be stored per user. And might not even be enough to prove a player even had an item. Lets say the pet's backup data had the enchanted tailcap in it. Sure, that would prove that between the current and backup saves you owned and somehow lost a tailcap with x enchant and y colors. But it wouldn't tell how you lost it unless there was also a saved record of every transaction your character did as well. Did you maybe trade the tailcap away, mail it, or drop it on the ground? Even more info that has to be saved. And possibly investigated in situations like this.
My guess is that the GM's don't actually investigate anything at all. If someone tells them they lost a tailcap due to the crash, they probably just confirm "yup, there was a crash, and people report losing stuff in these crashes. here, have a tailcap. derp." I wouldn't be surprised if someone could report the loss of something much more valuable (say some gacha dress or other fashion item) even if they lost nothing at all, and have a GM say "here ya go. derp."
Maybe the log only contains "transfer item id xxxxx (the id of your item) of yyyyy (the id for tail cap) from animal zzzzz to character aaaaa", but does not contain the actual stat for item xxxxx. So they couldn't know which kind of tail cap it is...