From the insights tab, with a date range of the past year, the state where the second most ad money was spent was California (after Pennsylvania). California is not even close to being a swing state, afaik?
As the most populous state, California has a lot of political donors - likely the most registered members in a state for both major parties. 1 in 8 Americans are in California. Those many small-value & high-roller donors help finance the swing state operations, but need to be activated. Donors are why both Republican and Democratic party candidates held events in California, when it's not in play.