The same is true in the US. Most (all?) states have state-wide speed limit "defaults" for town/city roads (i.e. 25 mph), highways and rural roads (i.e. 55 mph) and freeways (i.e. 70mph).

Instead of having a speed limit sign after each and every intersection, they're placed periodically. If you enter a road and there's no sign, that's the speed limit. If there's a different speed limit than the default, and you cross through an intersection and there's not another sign after it, that means the speed limit reverted to the default.

It can be a bit confusing (MN has 35 in city roads, WI 25) but also handy (wide open plains states often have much much higher freeway speeds).

The UK does have default rules, for example if there are houses directly facing onto the street (no front garden or similar maybe hard to imagine in most of the US but common in some UK towns) then the limits are low, if there are no houses at all the default limits are much higher. You are taught some rules of thumb for this when learning to drive. The posted limit signs are in addition to these rules, though they're more obvious.

But the tiny roads are usually where there is no housing - hardly anybody lives there so even the single lane of tarmac is a great expense considering average traffic. The "No housing => faster" is part of why there aren't signs limiting them. It's still a terrible idea to do 60mph though, just not necessarily illegal.

The difference is that in the US the defaults are jokes nobody abides by whereas in the UK they are, in some cases, numbers the "perhaps not lowest but normal person on a normal day" denominator will not find themselves wanting to exceed in basically every location where they apply.