I prefer just choose the username and the password and done, no need to verification your email to login and gamble. Maybe you can add with "if you verification your email, you can get an email for bonus or something like that in the future". For fiat sportsbook usually has more complicated signup method than bitcoin gambling website. On bitcoin gambling website you can signup easily.
Having mail address is a good thing because if we forgot our login details we need to recover it,for that we can just click the forgot password then we will receive the password resetting link but if we only have username and password the recovery process might be complicated since you need to wait for the support team to respond.
People who prefer only username and password may be not interested in depositing money but just play with free money from the site and next visit they want to use another new login to try their luck.
I agree that site should allow users to enter the site with just by entering username and password but later stage if a user likes site then they can enter like email, 2fa and other details to secure their account.