I still wonder if machines should be designed to deal with this, and would still be tempted to call it a "warranty issue" that they didn't handle a scenario they're clearly aware of.
On the other hand it would also be very nice if software just didn't decide it can unsuspend a laptop whenever, and then apparently do hours of work without ever re-suspending, so... I guess you could say I've got enough blame to ladle around for everybody.
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.
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.
The really hot stuff have sensors themselves, so if anything else is hot it will be revealed in the exhaust
If one puts a massive load on a CPU, the temperature spikes quickly and widely across enough of the die that the sensors work as expected and the system throttles / shuts down.
If one puts a small load on the CPU for a long period of time, areas that are not covered by sensors may overheat, damaging the CPU. This is true for other components in the system, too.
If the laptop is out in the air, convection will remove enough heat to prevent these invisible "hot spots". The bag acts as a blanket. Over hours, the laptop cooks parts of itself to death[0] in ways the system cannot detect.
[0] Or damages them in ways that make them unstable.
Edited to add [0]