Some states follow Assumed Maximum Posted Speed (in certain places) and others are Absolute Maximum Posted Speed. It is not absolute in an Assumed Maximum Posted Speed state that driving faster than the posted speed is against the law and deserving of a fine, merely it is prima facie evidence that you were driving dangerously but can be challenged and overturned. For example, in Minnesota, outside of municipalities on highways (there may a few more qualifiers like posted speed is 55 mph or higher and might need to be a divided highway, I don't remember 100%) an officer can pull you over and issue you a ticket for merely driving faster than the posted speed limit. You can even admit you were driving faster (I don't recommend this). You can still challenge the ticket in court. If you can convince a judge that your speed was safe, the judge can let you off. If the weather is dry, temperature moderate, visibility great, no other people or vehicles around you, you were able to safely slow down, and (prima facie evidence) that you posed no risk as no one was injured by you driving faster. In Wisconsin though they are an Absolute Maximum Posted Speed state so if you are found to have been driving faster than the posted speed limit, that's enough to ensure you can be fined.

In Washington state, the State Patrol has gone on the record multiple times and with the media saying their enforcement is based on "Under 10 over", i.e. they won't pull you over for doing 69 or less on the Interstate in a 60 zone, or 79 or less in a 70 (assuming speed is the only issue).

Are you asking (1) why I think what I think?

Or are you asking (2) how we wound up in this situation as a society?

(1) I think what I think for several reasons. Basically everyone speeds. Probabilistically that includes he very lawmakers writing the laws, the police, and the road designers. I've also read some articles talking about road design, and in it it's mentioned that the designers factor in that most people will speed if the road conditions are amenable. I've also seen police cars driving around without their lights on, passing people at higher than the speed limit, and when unable to pass, the appear annoyed to me.

(2) I think this situation arose in sort of a "normalization of deviance" manner. Police didn't want to be too strict, or didn't want to bother fighting tickets for people speeding only a little, so only gave tickets for people speeding a lot. Then over time many people realized that, and started speeding a little. More are and more people started speed just to fit in with the surrounding traffic, until eventually everyone was speeding. Peer pressure. I've heard driving the same speed as the surrounding traffic is generally safer than driving significantly slower (or faster). Once everyone is speeding, that includes lawmakers, road designers, and police. And they factor that in when they write laws, design roads, and enforce laws.