We considered keeping an internal balance but it breaks some very legitimate use-cases for ERC-4626.
IMO, being able to donate to all the vault shareholders is a feature, not a bug. It can be combined with things like a vesting contract to create “staking” incentives. Also, it raises the question of what to do with tokens that are held by the vault but not accounted for. Are they lost? Do we need some governance to recover them?
Adding asset tracking on top of our code is decently easy to do by just overriding a few functions. Devs can opt into that (we even considered providing this directly). But if it was integrated by default, and someone wanted to opt out, they would likely still have to pay the gas cost of updating something they don’t use.
That is why we did not go for that option.