Great question. Today's waveguide technology requires flat glass. (This is public information - Karl Guttag's blog covers the topic very well). Product designers have to either make the glasses flat (like Spectacles V4) or make the glasses much bulkier to hide the flat waveguides (like the front of Magic Leap). If added bulk were just a question of size, maybe that wouldn't be such an issue, but for a head-worn product every single gram of mass is important to optimize away.

Thank you