This answer is really too easy to guess:
Underground space engineering consumes a lot of manpower and resources. Even companies have to weigh it carefully.
Does Rebirth Nightclub make money? It does make money, enough for Rogel to earn a fortune, buy one of the few globally limited supercars, live in the most luxurious villa in Night City, and enjoy the world's finest wines.
But underground engineering is another matter entirely, and it's not something she can afford. Furthermore, there's no need for Rebirth to expand downwards.
The architectural space information Lille hacked from the Rebirth Nightclub network also indicated that this expansion was definitely beyond Rogel's capabilities. From every perspective, it was unnecessary...
So the reason for the expansion could only be because of V and Jack. However, those two can't possibly think of this: why would they need such a huge underground space?
