It doesn't help society for everyone to travel at 90mph. Faster speeds are deadlier full stop, denial of this sort of thing is as asinine as climate denial or creationism, it's an already established fact accepted by anyone who studies traffic statistics.
Unfortunately the relationship between speed and energy is unintuitive (square rather than linear). Increasing speed from 60mph to 70mph doesn't sound like much but it increases energy by over a third.
In an unachievable ideal world instead of a speedometer and speed limits we would have an "energy meter" and energy limits, which would also take into account the mass of your car.
You are only considering safety and discounting everything else. The capacity of the road network is absolutely related to speed. Slowing down transportation across the board amounts to slowing the economy, which has it's own health impacts. It's not obvious what the ideal balance is. For certain, 1 fatality about every 100 million miles driven is pretty low, especially compared to the sheer amount of utility we get from all of these cars.
You’re not considering glaring effects too. Wind resistance increases with the square of speed. Going 85 instead of 70 or even 75 is a significant change in fuel economy.
