Well, there are systems that do this. Many new cars have speed limit sign recognition built in and they will display the current limit on the dashboard(and sometimes show you an icon indicating that the vehicle thinks you are speeding).
The reason why this is not more aggressive is because it's only seemingly a simple problem - in reality it's far far from it. In my experience of driving around the EU - I had a Mercedes CLS(brand new 2019 model) with adaptive cruise control and speed limit signs recognition, and while amazing when it worked, it worked maybe 50% of the time. Issues I noticed:
1) it kept detecting speed limit signs on roads parallel to the one I was on [0]
2) it kept detecting speed limit signs on side roads - leading to actual dangerous situations, because I was on a 50mph road, going past a 20mph side street and the car would suddenly hit the brakes because it detected a 20mph speed limit. That's 10000% unnacceptable in a production vehicle. [1]
3) it kept reading speed limit signs on the slip roads leaving the motorway - so you're doing 70mph on the motorway, and it suddenly reads the 50mph speed limit that's for the motorway exit. Same problem as with point 2.
4) In situations where there are no speed limit signs, it would use the map data - and even though this was a brand new, March 2019 vehicle, it had no idea that a stretch of motorway next to my house is not limited to 50mph anymore, and hasn't been for the last 2 years.
In practice, I had to keep overriding the speed limit chosen by the car constantly, and after few days I switched it fully off as it led to me nearly shitting my pants as the car hit the brakes for seemingly no reason whatsoever few times.
And then I have concerns about countries where the speed limit is only valid to the nearest intersection - and the definition of an intersection varies. There are paved roads merging with the main road which are not intersections because the side road leads to private property, and then there are dirt tracks which are intersections because technically it's a public road open to motor traffic. There are no signs to indicate either - the driver is just "supposed to know". It's idiotic, I know. And in some countries(Poland) the speed limit can change as often as 4-5 times in a span of a single kilometre. What's even worse is that there are situations when the speed limit can actually increase(!!!!) the speed limit set in a certain zone(built-up zones have a 50km/h speed limit, but this can be increased by a regular speed limit sign - and then yes, it goes back down(!!!) to 50km/h after an intersection). All of this stuff would be incredibly hard to code into a system like this, and it's one of the reasons why I suspect the proposal made by EU to have automatic speed limiters built into cars will either fail, or people will just keep disabling it altogether, making the whole idea pointless.
[0] Here it would think the side road has a speed limit of 50mph, because it would read the wrong sign:
https://www.google.pl/maps/@54.9929676,-1.5788994,3a,75y,78....
[1] Here for example the 20mph sign is placed at an angle to the road, so the car would read it going past it - and set the adaptive cruise control to 20mph which is incorrect for this street:
https://www.google.pl/maps/@55.0074774,-1.615172,3a,41.3y,90...
Same here:
https://www.google.pl/maps/@55.0071611,-1.6214135,3a,75y,240...