Allow me to add to this:

It's incline and human laziness. Earth isn't flat but has stuff on (and under it), that is also a reason why we dont have vertical roads (even if we can build them).

In engineering, there is this thing called rise (basically a tangent) it's used to figure out distribution of force across some incline. So in the same way that climbing a 10 m of ladder is harder than walking 10 meters straight, getting up 5 meters in height over 10 m, asks for more energy to compensate for force distribution.

We peeps are very, very lazy. I mean I could go to the village and get cheap and organic produce, but that commerce store is right next to my door. So in the same way we dont like to use roads or go places that are more difficult than walking straight (and it isn't safe for trucks and other vehicles with low centre of mass). So if you need an incline of 5% for any pass to feel easy, that means that for an elevation of 1m, you need a ramp of 20 m. Now that is the bummer about inclines, because to get to 1000 m elevation you need to have road so long, that no neighbouring city can use it (ie 20000 m) - so we tend to pack it into twisty and turny shapes because if we dont there is no point to it.

So engineers tend to design stuff in such way that if you gonna use it, it will be easy... and even if you feel like a daredevil, chances are low enough that you will hurt yourself. That brings the research mentioned above into the topic.

See, we are lazy but also easily bored (listen I never said humanity was easy or simple). To put it in eli5 terms ... more nothing changes, more your internal race car driver wants to take a wheel. So even if we make it so that, if one wants to go daredevil, they are 90% safe... more time they have on such road more daredevilly they will become and more they go into that 10%.

In actuality, falling asleep has no correlation with road length, but rather increase of speed does. So we tend to make visual, auditory and inner ear stimuli at around 30 min marks at every road (due to this). This is why roads have speed limits, and why they are measured in hours. If you are going 60 mph at 30 miles you will have some tree, some sign or something that resets your inner daredevil.

So to climb up/down - we need long roads, to make them fit we make them sort them like straws. Length of a straw depends on amount of time where people need a stimuli refresher.

If anyone has inner ear problems during non straight roads.... it's not the roads it's the driver. You can always drive 30 in 20 zone, but those stimuli are passing 1.5 times faster. To some their passivd attention has no effect to their inner balance - ie they don't register set stimuli. For others, it does so their brain gets lots of inputs. It's like trying to listen to audio book while watching unrelated movie and tooning in on your neighbours fight - while you can't get away because it's a moving vieachle.