It doesn’t really make sense for things within a canvas to have their own set of Properties.
For the same reason a note section wouldn’t have Properties separate from the whole note.
And how would cards with their own set of Properties show up in search when using the [property] search operator?..
Plus, as OP has noted, there’s a workaround available of either embedding proper notes onto a canvas instead of cards, or transforming cards into proper notes.
So I don’t think this will (or should) get implemented.
But I’d like the .canvas file itself to have properties; that seems doable.