I think manually is a common concept in casinos, there has been a lot of people who have abused the automatic system with the double spending method of nonconfirmation with zero fee etc etc which goes back to the sender because the fee is not enough and it is not getting picked up by the blocks.
It means if casinos let the automated withdraw work, they would see the money coming in and then they would credit the gambler and if they lose they get their money back anyway, if they win they both get their money back plus the wins. Hence the automated system doesn't really work anymore, maybe if the confirmation process takes longer, like takes 3 confirmations to get credits but that would mean gambler can't gamble right away and their gambling hype may die.
Of course manual processing is needed especially for big withdrawals. I actually hardly make any small withdrawals (never less than $50) so I always make my withdrawals carefully and on a significant amount,,, and it the site's hot wallet is big enough I get it instant (Talking about yolodice and bitvest for example).
If not, admin just has to do it manually for big amounts. VERY normal.
while I do agree that manual processing of big amounts is a must ( to check if there wasn't anything shady going on )
generally , manual payouts is what makes gamblers reconsider playing at a casino
nothing could be worse when you que your coins to get paid and they do not arrive to your wallet
makes all kind of scenarios run through your head , you start to ask people in chat , worry etc.
and doublespending is not a thing anymore , there are no casinos that I know that accept instant deposits and frankly , there is no reason to do it
after your deposit gets one confirmation , you cannot doublespend , not possible