There’s 3 answers.

1. It’s shitty to maintain underground wiring and it’s shitty to trench it in the first place, so while you are saving money in the long run, you’re paying upfront to run it and your routine maintenance costs are higher in short bursts instead of lower cost per issue, but a lot more issues. This makes people angry the money line didn’t go up enough that month and now you’re fired… even though you saved them money in the long run.
2. You can’t run it very far underground. I won’t get into the physics, but suffice it to say, running power underground tends to be “leaky”, and it will bleed away very quickly over relatively short distances.
3. Sunk Costs. Tech for trenching and boring has gotten a lot better, it costs less to do than it did 100 years ago, but moving the lines now and tearing down the old ones would cost money upfront and no one wants to pay for that.