My 1968 built glider has an L/D of 44 with just under 18m wing span and average chord of about 0.5m. A more modern 18m design will have L/D around 50.
To make this work, it has only one seat (i.e. pilot only) with just enough room to wriggle in flight, and no sticking out bits like engines to spoil the aerodynamics.
Scale this up to take a few hundred passengers, and you can see why it's not a commercial proposition.
Plus, the trade off for the high L/D is that high speeds become impossible because the tips start to produce down forces (amongst other complex engineering and aerodynamic problems).
BTW, ETA is not really a powered aircraft but a self-launching glider with a fold away engine. The claimed L/D of 72 is only achieved in glider configuration. Only 2 seats.