Atlanta, GA has underground power transmission through much of the city (and specifically my neighborhood within the city) and we can still lose power in storms. What isn’t underground currently is being moved there in much of the city. However, the transmission equipment is still outside and still has physical limits. There’s only so much water, so much wind, and so much lightning electrical systems can take without someone going dark. Underground trunks can also flood and short so they’re not a full panacea in areas that have heavy storms.

We’ve had a mild storm season the past two years and we’ve maybe been down seconds in total in my area.

While the average I’m sure is correct, the distribution is going to depend a lot on what is going on with nature. When I lived in California I lived in an LADWP area so I didn’t experience rolling blackouts. As a kid I had friends that would come over because they lived in an Edison area and play time in air conditioning at my house was much more enjoyable in August. If you were to find the average downtime during that period of time I expect it would saddle everyone with 10-15 minutes of outages even though my area never went down and my friends lost power for a few hours each week during heat waves.