I wonder the same. Its a big up front cost as digging and drilling is expensive. After that I think it should not be an issue. Ventilation sure would need upkeep but given the milder temps I think savings vs additional ongoing costs should favor less cost at that point. It should be able to be done to not allow water to get in unnecessarily but it might make flooding worse above. Thing is except for deep mines we have not went that far underground. So like unless we did go that deep which would be that much more expensive or have buildings popping up to the surface then you would not have high rises. we would still be growing food on the surface which still takes a lot of land. My guess is it would just be better for people who don’t need to be way out like working or running in a farm live in a high desity urban area and get rid of the wasteful in between spaces.