A lot of modern construction does use underground lines because they are far more reliable overall. But they're also a lot more expensive and a lot harder to service when they do break, especially if they run under pavement or other manufactured ground cover. While today underground lines are feasible, power lines predate widespread availability of powered construction equipment, so the amount of digging required would have been prohibitive. Now the biggest reason we still use overhead lines is simply institutional inertia - it's what's already there. Replacing the entire grid would be cheaper long-term but has an enormous upfront cost and doing it piecemeal would require transitions from overhead to underground that introduce a whole bunch of engineering and safety issues. There are a lot of people who think we should make the transition anyway for the exact reasons you brought up, and in many countries underground lines are now standard.