As far as I can tell it almost exclusively follows the existing roads in Europe. Probably an easier way to secure rights in one go like rails used to be for telco lines.

Pipelines, Sewer systems, rail too.

Geography and property lines, twin banes of every infrastructure builder.

And because getting heavy plant up through a mountain range to dig trenches etc is a pain in the arse. It’s easier to do it along a main road.

If you were smart about it then you wouldn’t go up in the mountains in the first place. To get the lowest possible latency you want a straight shot, so obviously you should just bore your way through at the level of the plains on either side.