Hmm good points, but you also ignore the power costs here - using oscillators, open pins as thermometers and rf antennas requires additional power draw, not to mention modifying these devices may be nearly impossible due to their embedded nature.
Even presuming you modify the hardware/firmware, the additional cycles to handle and process the sensor data mean additional power draw compared to normal operation (embedded devices may frequently power down to wake from some interrupt to save power, additionally not all instructions turn on the same number of transistors - floating point ops require more power than simple branch instructions) - something again that prohibits doing this easily.
So "without any added cost" is simply untrue.
The reality is that randomness is relatively expensive, whether via hardware or software. Phones have more sensors - they also have massively complex SoCs and large batteries, which still are drained often over the course of a single day. They also tend to cost 1k+ USD, at least for flagship models (prices go as low as 50$ these days, but this is more from economies of scale and resale value economics than because phone hardware/software is suddenly cheap to manufacture)
Even presuming you modify the hardware/firmware, the additional cycles to handle and process the sensor data mean additional power draw compared to normal operation (embedded devices may frequently power down to wake from some interrupt to save power, additionally not all instructions turn on the same number of transistors - floating point ops require more power than simple branch instructions) - something again that prohibits doing this easily.
So "without any added cost" is simply untrue.
The reality is that randomness is relatively expensive, whether via hardware or software. Phones have more sensors - they also have massively complex SoCs and large batteries, which still are drained often over the course of a single day. They also tend to cost 1k+ USD, at least for flagship models (prices go as low as 50$ these days, but this is more from economies of scale and resale value economics than because phone hardware/software is suddenly cheap to manufacture)