Because torrent clients can't handle that many pieces in a single torrent. There are algorithms that are super-linear, maybe even quadratic or worse. They start causing trouble int eh TB range.

Also the UI for adding many torrents is much nicer than for selecting a non-trivial subset of files inside a single torrent. Also many parts of the ecosystem handle partial-seeds that do and will only for the near future seed a subset and not leech any other parts. They often get treated as leechers, despite not really being leechers.

TL;DR: 2k files are just a watchfolder and a cp * watchfolder/ away from working. Scaling does not work with one fat 32TB, however.