There are also some of us who didn't have the choice
IIRC for the first few years NTL's STB's didn't have built in cable modems (when I signed up we had to pay something like £150 and buy our modem from the approved dealer).
These days as patrickp says it's likely to be because people are on speeds the STB's don't support, or I guess people for whom an STB's modem has been problematic