Because it varies from state to state, most of us don't live in the same states we were born in/grew up in, and politicians keep changing them anyway. In the past five decades, the Federal highway speed limit has changed from "Whatever each state decides is reasonable and proper" to
an idiotic mandated 55mph in 1974... (Internet regs require me to embed this now)
... to a grudgingly raised 65mph in 87-88 when they realized people weren't going 55, and then finally in 1995 back to "Whatever each state decides is reasonable and proper" when they realized that 65 wasn't working either and the NMSL had accomplished exactly zero of the goals it set out to do.
Then even within the state, or even in a city, they can change it. In the last decade alone, the Dallas North Tollway has changed its maximum system speed limits at least three times that I can remember - to 65, 70, 75. They're talking about changing it to 80 in some of the new rural extensions.
Texas doesn't have a "unless otherwise posted" speed limit for highways, as is the case for most states, especially Western ones; road users rely on posted signage. This is normally just fine, but then there's the case of State Highway 121 on the northern edge of the Dallas metroplex. It was recently rebuilt as a fully modern interstate-spec highway from a two-lane rural highway but the contractors forgot to post speed limit signs. It suddenly became the 121-bahn with no effective speed limit and no statistical change in accidents, but about a year later someone complained about the missing speed limit signs and ended the fun for everyone.
Even in states that have "XXmph unless otherwise posted" regulatory highway limits there can be loopholes (mostly created in the era of the stupid 55mph limit as end-runs) that render such limits useless. I checked California's basic speed law and their current "unless otherwise posted" is 65mph, with 70mph only if posted. Two lane undivided highways are 55mph under this reg... but then you find out about California's 85th percentile rule. Because of California Vehicle Code 21400 and the case law that has arisen since it was implemented, if the 85th percentile speed of people on the road is (say) 75mph as determined by regular survey but the speed limit is a posted or regulatorily determined 55mph, any ticket written for 'speeding' between 56mph and 75mph is thrown out upon presenting the ticket and the latest traffic survey to the judge - basically making the posted speed limit useless. Something I exploited ruthlessly when I lived in CA.
Cities in some states can impose their own additional limits, which can be hilariously disharmonious - see the example of the cities inside Dallas that I posted above; there's one point where the limit is a posted 50mph (as I recall), but if you cross an intersection (because the intersection is on the border) you are suddenly confronted without warning by a 35mph limit sign. I'm sure you can guess where the cops like to camp out. New York City has imposed a 25mph city limit, I'm sure prizrak can tell you how frustrating that is.
How are we supposed to know statutory default speed limits that may not even exist in a state? If they do exist, they may have giant loopholes you can drive a Death Star through, or be changed by locality. This is why American road users rely on posted signs - there simply is no consistent national framework of regulatory speed limits and it varies not only by state but by county and even city. The regulatory limit can actually be four different numbers at an intersection, one per side.