I noticed that armrests on all the flight's I've taken recently can't be lifted on aisle and window seats (the edges of a row).
Last time I flew, I got lucky: I was alone on my row and as such I was able to lay down to sleep. However, it was hard to make myself comfortable as that armrest was pressing on my spine when I tried laying down with my back to the window.
Also, it would be much easier to get out of my aisle seat if I could lift the armrest. I'm a pretty tall guy, so I always feel like a contortionist trying to get out of my seat.
What's the problem with letting people raise the armrest on window and aisle seats?
I'm guessing it's some sort of safety issue, because the mechanism seems to be the exact same one as the other seats, just with an extra locking mechanism added.