Civil engineering is a very complex thing, which also has been around. A lot of the things they due is because of lessons learnt in the past. If you have questions like “Why don’t they just?”, I’m sure an actual civil engineer with knowledge of the situation can tell you exactly why. It’s all much more complex than you could imagine.

I’d suggest watching some of Grady on Practical Engineering on YouTube to get a basic idea of how much goes into projects such as designing and building a road. In my experience a road contains a lot of infrastructure around and underneath, way more than you’d assume.