Why is there a delay in delivering collectibles after a drop?

When you purchase a collectible, it needs to be "minted" before it is delivered, and this can take time.

If you received the "green truck" confirmation screen, a red "Pending delivery" banner on the collectible screen, and/or your gems were deducted from your account, the item is being minted for you and could take up to a few hours to be sent to your account. Please be patient while the system is allocating collectibles.