A PC case is like an oven, but much less thermally insulated, so part of what I'm about describe is less about being thermally insulative and more about being a sealed vessel.
Your kitchen oven is designed to continually create heat inside a sealed box, which is why if the door opens, oven temps drop suddenly (assuming you have an oven temp probe to see this occur). A PC without fans or airflow is much like this - the PC continues to generate heat inside the box, but if air cannot remove the heat, it continues to collect, warming the internal air more and more, faster than it is expelled. Even small PCs or machines with low TDP will still do this, although at a slower rate. Having good fans is like opening the oven door and leaving it open - the oven will never get hot because heat can easily escape the sealed unit.
The same principle works for the misguided concept of 'put the computer in a refrigerator or freezer'. These are both sealed containers and are well insulated. Even though they have cooling components to chill what is inside, they are designed to only do it ONCE, and then maintain lower temps. PCs continually put out thermal load, which easily builds beyond the heat load a standard residential fridge or freezer can remove and the compressor will run continuously until it burns out, all the while really failing to do a good job of cooling. With a sealed refrigerator or freezer, you end up with the oven scenario, where the PC is inside a sealed box without a way to vent heat.
Having a PC with good airflow is more important than the expensive cooler you have atop your CPU because at the end of the day, if that $100 heatsink or $200 AIO can't get cool air, it isn't going to cool effectively.
Edit: I should also clarify that any PC is not 100% 'sealed' or airtight, but the idea here is that there is more air continuously circulated internally than being exchanged with cool, fresh air from the room.