Think of it in these terms. Each thing consists of two parts
- “accidents” (appearance, taste, physical characteristics and so on)
- “substance” (something intangible that makes a thing what it is or a person who they are)
For example - look at yourself.
On the one hand your physical characteristics are probably completely different in every respect to what they were when you were a baby of one hour old. In fact every cell in your body would have been changed many times since then. Yet you retain your ‘substance’ - whatever intangible property it is that made that baby fuzzybunny the same as the grown fuzzybunny.
On the other hand people who do have every physical characteristic exactly the same as each other do exist - they’re called identical twins. Even twins DNA is EXACTLY the same. So their ‘accidents’ can be exactly the same. If a clone is made of you same deal. But that twin or clone doesn’t have your ‘substance’ - fuzzybunny’s twin or clone isn’t the same person as fuzzybunny.
So every person, and every thing, has properties beyond and different from the physical that make that person or thing what it is. That make you you, not your twin or your clone or a perfectly physically correct robot replica of you. And that make you you even if you had plastic surgery to totally change the way you look. This unique property is what we’d call ‘substance’.
So it is with the Eucharist. The physical and perceptible qualities are those of bread and wine, but the ‘whatness’, ‘substance’ or ‘essence’ of those things becomes the ‘whatness’ or ‘substance’ or ‘essence’ of Christ instead.
As for a deeper ‘why’ - well, Christ’s substance is the substance of God - what sort of appearance exactly do you think His Body, Blood, Soul and Divinity would or should naturally have?

:hypno:
It wouldn’t be an appearance we could comprehend, and certainly nothing we could take into ourselves as we do the Eucharist, yet He commanded us to do so. So in His wisdom he gave us something that’s easy for us to see, touch and eat and drink.