Because the hashes that are stored in the .torrent operate on that unaligned data.
In practice, what this means is that you can't verify that two files of the same name and size but at different alignments within the consolidated data stream are identical; you can't compare hashes, can't do anything without first downloading. This opens the door to mass poisoning of swarms without even having to enter them in the first place.
There are potential solutions (including providing a broader hash per-file, as opposed to per-piece), but my statement was only that it's not that simple, not that it's impossible.
Why do you want to be completely backwards compatible with classic Torrents? Torrent2 can dump some features of classic torrenting, like folder structure, and mandate that each "subtorrenat" is basically a single Torrent1 containing only 1 file and no folder structure.
In practice, what this means is that you can't verify that two files of the same name and size but at different alignments within the consolidated data stream are identical; you can't compare hashes, can't do anything without first downloading. This opens the door to mass poisoning of swarms without even having to enter them in the first place.
There are potential solutions (including providing a broader hash per-file, as opposed to per-piece), but my statement was only that it's not that simple, not that it's impossible.