Perhaps there will be some insight if you look at it historically. The Captains Club happy hour was in the back long before there was such a thing as a beverage package, when all drinks were purchased one at a time.
In those days, the happy hour included passed hors d’oeuvres, that took time and effort by kitchen staff and waitstaff.
On turnaround day, there are almost always dozens of new staff members who join tge ship in the morning and have precious little time to settle in and jump on their new duties. Some barely know their way around the ship. Having a slightly lighter load for the first evening made perfect sense from a staffing point of view.
Also, since it was available to Captains Club elite and above only, it gave the Captains Club host a little time to determine who was eligible to attend. It became especially tricky in the years when paper coupons were in use. These were put in your state room but in the case where a stateroom might have been missed, it gave the Captains Club host a day to fix the errors, and a happy hour started on day two.
Move forward to the introduction of beverage packages. Everyone who wanted a beverage would be purchasing one, giving the opportunity for the bar staff to sell a beverage package.
Today, for anyone who has a beverage package included in their booking, the happy hour is probably superfluous— Whether it is offered on embarkation day or not makes no difference to them.
And for those who do not have a beverage package they would be buying their drink that night s has always been the case.
So there really isn’t any incentive or advantage to Celebrity to change. It has always been thus, and probably always will be.