Suppose I have 1 BTC.
I send my 1 BTC to an address XXX that has a balance of 50 BTC.
Next, 5 BTC are sent from XXX to YYY.
Where is my 1 BTC now? Does XXX or YYY have it?
The correct answer is "there is no such thing as a bitcoin, so the question is meaningless".
This is by design as it enables proper fungibility, which is maybe the most important property of a currency (as opposed to just an asset).
This is the whole reason CoinJoin, for example, makes any sense. Inputs 1,2,3 and outputs 4,5,6. You cannot deduce which input paid output 6, not because it is hidden, but because the question doesn't make any sense (well, technically there is SIGHASH_SINGLE, but let's ignore that).