An average long haul economy seat costs 500 (for arguments sake), and they could charge 80 quid at most for extra legroom. The numbers don't really stack up. Anything over ~5" extra starts to cost money.
Airlines like AY and AA do it because the emergency exit placements don't allow an extra row, so they may as well stretch the front few and charge extra, but BA seems adept at squeezing seats in right up to the limit.