So that someone doesn't write an app that polls at 20+ hz and burn the battery. Apple is optimized for user experience and simplicity, that includes precluding bad behavior. If you want a real time weather station, a smart watch is not the right tool.
My hot water kettle heats water, why can't I wire it up to be my whole house heater?
I mentioned it specifically because I write software for hobby devices that poll at 20hz that use literally the same Bosch sensors as apple. The use case is for gliding variometers (audio altimeters)
The sensor that is in the Apple Watch draws significantly less than 1ma when polled at 20 hz. Without an EE degree I have my devices, including the 90s era processor and piezo speaker, running for 100+ hours on a 150mah button cell.
I cite this example because I KNOW what is possible. This is a pure software issue.
I suspect that apple rate limits because the raw sensor data is quite noisy, and would look glitchy in a badly designed app. But there is a lot of signal in that noise that I want access to. Instead people in the gliding hobby spend hundreds to buy devices that have the same sensor package as an iPhone 6, but are able to access the sensors in a way that are useful.
My hot water kettle heats water, why can't I wire it up to be my whole house heater?