Because software complexity is truly out of control. Everything needs their own firmware and software and periodic housekeeping tasks to prevent the system crashing. No one knows or has enough time to account for all of those problems, let alone just to build an interpreter environment.
I think Apple and Google both using their phone OS for smartwatches is one indication of lack of such capacity in the industry: Apple Watches come with ~300mAh batteries, old Nokia had 500-700mAh. Watches are rated for 36 hours in normal modes or 72 hours in low-power, whereas Nokia would ran for minimum 1 week, despite ~15 years of improvements in semiconductors.
We'd have to wait for maybe GPT-8y or DeepSeek-R4, probably on wheels, each given a cubicle to get it back down to human scale.