Because the central limitation is still mass; a friction-fit frame light enough to sit on your face all day without fatigue is the market expectation for eyewear, and devices generally require parts which weigh more than glasses.
Basically you have about 130mw power budget. You need to do position, display, sound output, transcription, etc.
https://www.secretbatcave.co.uk/software/musing-on-openais-d... << an explanation here
for assistant glasses to be useful, you need to know who, what, when, where for each operation that the user cares about. In the experiements that I ran, that took about 35 watts continuous on the headset (using a jetson) and at least 2 big GPU machines in the backend. This was in 2024, so perhaps its not quite as heavy now. but its nowhere near 130mw