We used to joke that it's because the original Anatomie Studio was 7.5 metres long. The real reason for this is that you want the ropes to be about four times your arm span so that you can always pull the ropes through a space in just two smooth movements. It used to be very popular to tie with 8 metre ropes, but this is in fact quite long for most purposes. You generally want to minimise the amount of rope you pull-through when tying in order to help maintain decent tension and speed. We do sell custom lengths of rope as well for those who prefer them though.