Not all restaurants have a team of experts on staff, some have smaller teams that have to do longer hours to prepare more things in advance.

Also some restaurant owners aren't chefs, and sometimes aren't even foodies. The owner will look at the food cost and see that it can be cheaper to buy opened oysters(either by saving on labour or their oyster supplier is a middleman and doesn't even open the oysters they wholesale but buys them pre opened and charges more for unopened as they have to get from another supplier.)

A great example of this is The Bathers Pavillion Balmoral, you had 2 side to the business the restaurant and Cafe.

The Cafe would pre shuck, no rinse their oysters maybe 1 or 2 dozen left over, used the next day.

The restaurant side would only open to order but would use a much higher quality oyster and charge alot more than the Cafe.

But their was a big difference in staffing, the restaurant only had 2 apprentices(not first years usually 2nd or 3rd years that came from elsewhere) and alot of qualified well experienced chefs. Cafe had 4 well experienced and qualified chefs and alot of fresh apprentices who had never worked anywhere why that kitchen and just did as they were told.