Most sites, at least dice sites, provide either a login url (Just-Dice, SatoshiDice) or ask for a user name and password (BitDice, PrimeDice).
One of the benefits of Bitcoin gambling is betting anonymously.
There has been a couple of sites however that allowed you to login with your social media profiles, though they seem to be dead now

Do you think sites should offer the option of login via social media profile as well as the normal username/password?
it does not seem necessary because many gamblers want to hide their identity while gambling. in my country gambling is illegal so it would be better to create an account on the gambling site anonym. but if the gambler likes that option because there's no need to create an account on that site which is time consuming and just needs to connect it to a social media account it's good too.