They're sold to people at a discount relative to Netflix's standard pricing -- e.g. $20 (or BTC equivalent) for 1 year of service. The customers don't always understand that they're paying a thief/fence for stolen goods.
Based on some forums it appears that often stolen credit cards are used to buy redeemable codes. Those codes are then sold for far less... Basically washing the stolen CCs.
Hacking individual accounts seems much more difficult / less rewarding financially.
You can buy the accounts for $2 or you can pay $20 for unlimited accounts. They sell programs that you click a button and it just gives you another hacked account from their database.