My apologies for not being clearer that this is not only about the CPU. It's about all the components in the laptop, not just the CPU. I used the CPU in the example because it's a component most of us are familiar with.
It's not that the sensors are in useless spots. They're the best sensors in the best spots the designers could make work for the price point they're trying to hit.
To answer your CPU question:
CPUs have thermal sensors, probably the highest number in the machine as they work with clock speed scaling and the CPU is in the top 2 most expensive components. But each of these sensors is covering a large area, which may not heat up evenly under low loads.
A heavy load is the easy case and the one they're designed to detect: the CPU core is fully loaded, it's generating maximum heat, and the entire core area will heat up quickly.
A light load is harder to detect. Depending on die design and sensor placement, one corner of a core may heat up in a way the sensor doesn't detect well at low CPU loads.
These sensors are also only accurate "enough". This corner of a core may have to get to 110C before the sensor realizes it's overheating and throttles it. It could sit for hours at 105C, very slowly toasting itself.
But CPUs are, generally, actively cooled or have large heatsinks, so this is less of an issue in practice... though wrapping a laptop in a blanket overnight might make it an issue.
To move on beyond CPUs:
There are many thermally sensitive components in a laptop. Some of them have thermal sensors. Others do not. Almost all of them rely on convection to shed heat. Leaving the laptop effectively "on" and in a bag overnight may bring them to a temperature that damages them.
Light load over a long period of time will result in a fairly uniform temperature distribution. It sounds like the system must have no "general" temperature sensor measuring the air temperature inside the machine (or chassis temperature).
Probably some small component on the logic board generates a bit of heat, requiring a certain "internal ambient" temperature, and the designers (never having thought to test it with restricted airflow) never noticed the implicit assumptions they had made.
It's not that the sensors are in useless spots. They're the best sensors in the best spots the designers could make work for the price point they're trying to hit.
To answer your CPU question: CPUs have thermal sensors, probably the highest number in the machine as they work with clock speed scaling and the CPU is in the top 2 most expensive components. But each of these sensors is covering a large area, which may not heat up evenly under low loads.
A heavy load is the easy case and the one they're designed to detect: the CPU core is fully loaded, it's generating maximum heat, and the entire core area will heat up quickly.
A light load is harder to detect. Depending on die design and sensor placement, one corner of a core may heat up in a way the sensor doesn't detect well at low CPU loads.
These sensors are also only accurate "enough". This corner of a core may have to get to 110C before the sensor realizes it's overheating and throttles it. It could sit for hours at 105C, very slowly toasting itself.
But CPUs are, generally, actively cooled or have large heatsinks, so this is less of an issue in practice... though wrapping a laptop in a blanket overnight might make it an issue.
To move on beyond CPUs: There are many thermally sensitive components in a laptop. Some of them have thermal sensors. Others do not. Almost all of them rely on convection to shed heat. Leaving the laptop effectively "on" and in a bag overnight may bring them to a temperature that damages them.