Well, for one, you have zero chance of being attacked by killer sharks with lasers on their heads.
And more usefully, a pool provides a controlled and familiar (and shark-free) environment for developing the dive skills and gaining comfort in the water.
Secondly, you can learn at your own pace – you don’t have a deadline of being in a 3-4 day course, or having your flight back home in a few days.
Lastly, this frees up more time on vacation to actually go diving and see sharks and other cool marine animals (yes, that is a thing and no, they won’t attack – divers go to great lengths to be in the water with sharks. It is an amazing experience).