In practice it is so much more complicated. You have different soils, some that are water permeable - in Boston (I think) they solved that by literally freezing the ground. Does that sound cheap?

Then you have issues going under buildings (which you always are - otherwise why bother tunnelling?). All tunnelling disturbs the ground. When they built the Jubilee line extension under the houses of parliament they had to monitor how far Big Ben started leaning, and do something call "compensation grouting" to tip it back upright (basically you drill horizontal holes under it and pump concrete under it).

Tunnelling is just expensive. That's why it only happens where the alternative - building on land is very expensive.