I don't fly regularly any more, but I did for several years (mostly long haul), and I found the same thing - even if economy was only half-full, premium economy was almost always full.
A side effect also meant it was almost impossible to use airline points to upgrade from economy, since the number of "reward" seats is very limited anyway, and always seems to be "zero" on popular routes.
At the time, I always wished they would increase their premium capacity at the expense of losing some economy seats, which I figured would have reduced the cost of premium economy a bit too.
Same thought I had. It's crazy to me they aren't expanding premium economy row by row til it's not filling up. It's often nearly double the cost, but doesn't nearly take up double the room. I think they'll actually find they can expand to infinity, as consumers are willing to pay money to not suffer.
A side effect also meant it was almost impossible to use airline points to upgrade from economy, since the number of "reward" seats is very limited anyway, and always seems to be "zero" on popular routes.
At the time, I always wished they would increase their premium capacity at the expense of losing some economy seats, which I figured would have reduced the cost of premium economy a bit too.