Others do not natively issue an ERC20 receipt, e.g., Convex. In these cases, there needs to be a method for accounting for the staked position, i.e., some of TokenA has left the vault (it is in the staking protocol), and we need to know how much. While this could be accomplished with an "external position," these positions still have the downside of non-fungibility (non-transferability, etc), and will not be available to funds adhering to stricter trust models.