Somebody with more engineering know-how can undoubtedly explain it much better, but there are significant differences in design requirements between a vessel that needs to contain internal atmospheric pressure and keep it from escaping out into the vacuum environment of space, and a vessel that needs to prevent hundreds of atmospheres' worth of external pressure in the deep ocean from crushing it. These are completely distinct and dissimilar conditions.
Kor
Yes!! Thank you, Kor, the Dahar Master!
While at first glance being airtight beats being watertight, she's a
Constitution class starship. A STARship!
There's a reason why we never heard of Starfleet starships landing on a planet until the
Intrepid class over a century later. And why in
"THE 37's", an excellent pilot like Tom Paris tells Janeway he has never landed a starship before, and why she says the same thing.
It's also why, except for ST09, we always see ships being constructed in space. Not on the surface... not by the mountains... not in a van by the river.
In space.
(And I know the real world reason we never got ships landing each episode in TOS, and it kept going through the franchise. Doesn't matter. It's part of the franchise's dna, and one of the things that sets it apart from others.)