Lack of shade + Too many windows that face east and west, vaulted ceilings leave little room for insulation, 2 large skylights also leave little room for insulation.
That's about it. There are things to improve, but they are all incredibly expensive.
It can also be 90 outside and less than 90 inside. It really is all down to how sunny it is.. which makes the windows the rather obvious culptrit. It's also why it can stay warm in the winter inside.. just has to be sunny and the house naturally heats.
Yes, technically all "insulation" issues, but just spraying more insulation in our tiny attic likely wouldn't make much difference.