And where would the heat sink vent to? If the camera is sealed, then the heat still stays in the camera. The heat sink can spread the heat around inside the camera body, but that only gets you so far before the entire of the inside reaches the same critical temperature. That the R5 is already so cramped, means that there is not a lot of space for the heat sink to work anyways.
Looking a bit further... you will notice that there isn't much space behind the sensor. There is that aluminum plate that is blocking (?) the heat to the sensor. If the heat were to be vented in front of the senor, then it would end up in the lens.Long story short; there isn't a lot of space to disperse the heat to, so really a fancy solution like thermal paste to conduct heat to a heat sink would be an expensive was of effort.
BTW, thermal paste is used to ensure heat connectivity between the CPU and the heat sink. Since a heat sink would be fairly useless, then thermal paste is moot. Some have mentioned seeing huge copper pads used a heat sinks which makes way more sense.