Modern car are perfectly capable of going 100mph on modern highways. Modenr highways are perfectly able to support traffi traveling at 100mph. Modern people are not able to react when Doris blinks to overtake the lorry infront of her, having previously gauged the car behind her to be "far enough back".
The issue is never that the cars or the highways limitations stops it from being safe, the issue I have with people who speed is that you're willingly involving other drivers you may pass in your behavior. About 32000 people in the UK are killed (1800~) or seriously injured (remaining figure) in traffic, highways/motorways make up roughly 6-10% of this number, the rest being attributed to rural and urban roads (headon accidents a primary factor).
The speedlimit in Norway is about 70mph on highways. I frequently see people doing speeds of 100+ during the day and they are traveling noticably very fast compared to other traffic. For the vast majority of the time it's not a problem, because I pay attention and I see that a car is approaching fast. But there are so many completely plausible scenarios where someones is being unattentive and you risk creating a scenario where you could crash.
So yeah, cars and highways can handle it. People however can not be trusted to not endanger others, so we have a speedlimit.