Many 55 roads were designed in what, the 60s, when cars couldn't handle or brake for shit and had the structural intergrity of a tuna can.
I'm sorry, but given how much cars have evolved, and how safer they are, speed limits nowadays are simply ways for counties/cities to make money. Limits should be a lot, lot higher.
If safety was the #1 concern, then emphasis would be put in building better-designed, longer-lasting roads, not on slowing people to accomodate how bad the road is.
Your argument could be valid if there were rigorous safety and maintenance inspections in every state, every driver paid perfect attention, and nothing could ever possibly obstruct the road.
MI, for example, has no vehicle safety inspections. There will be people driving around on corded tires with worn brake pads and failed struts.
Glancing down to adjust the radio, climate control, etc, (let alone texting) gets considerably more dangerous as speeds increase.
Going faster reduces your available time to avoid animals that go onto the road, debris that is left on the road, or slower/stopped vehicles. This is compounded by elevation changes and turns.
You also ignore that vehicles are only tested to survive 60-70mph impacts. Going even a little faster at 80mph increases your vehicles energy by 30% over 70mph. Good luck dissipating that excess energy and not splattering your face all over the windshield.
Get out of your fantasy land where everything is perfect and start considering your ideas in the context of the real world.