Not necessarily because no one lives underground and there are probably no existing things like property, gas lines, electricity lines, sewers, pipelines, roads, etc to avoid or reroute. And very little in the way of habitat.

The longest road tunnel in the world only cost about 100 million in the 90s for 25km so tunneling isn't always a gigantic Big Dig style clusterfuck.

In terms of legal complexity, it's fantastically easier than picking your way across and near thousands of individual plots of very expensive land owned by people with solicitors salivating at the potential fees, expensive private infrastructure, nature reserves and so on.

HS2 does not go through the complex geology of the Alps.