system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 Why do engineers blast through hills instead of building highways over them?

Can't get back to all the comments, thanks all <3

EDIT: I used HILLS and not MOUNTAINS on purpose lol, c'mon guys I'm not *that* clueless. Tunnels absolutely do not figure into my question. Thank you everyone for answering this has been enlightening.

EDIT 2: I don't think it has occurred to some browsers of this sub that the people asking these questions have probably considered their own question and come up with hypotheses already. Trust me, I had 2 hours longer to drive and think after I posted the question at a gas station haha. ELI5 has confirmed some of my hypotheses about this and also added many new pieces of information to the puzzle which I am grateful for. Some of you taking time out of your day to say "stupid question" or something along those lines, please don't consider becoming teachers, and go forth in this sub operating under the assumption that a lot of the questions asked here are not just asked out of curiosity, but also just seeking confirmation of hypotheses before going out into the world and spewing BS lol.
gpt
Roads have a slope limit, called the road's grade.  If the road is too steep, it will be unsafe.  Mountains are natural features, and they are often more steep than this.  So, to go over a mountain, you have to make a very long road to maintain the grade limit.  You can do this on the mountain, with a series of switchbacks.  Unfortunately, switchbacks are also dangerous and significantly increase travel time.  This longer road is also much more expensive.

At some point, the economics of tunneling through the mountain are cheaper than the long road in terms of construction costs plus all the time/fuel used to traverse the road.  Engineers usually build the cheapest solution, even when it's a tunnel through a mountain.