Even in relatively modern aircraft, the pilot, and therefore the seat, are an afterthought. The AW139 that I've been flying for the last 3+ years has a variety of different seats ranging from appalling in the short nose aircraft to just about ok in some of the newer machines. However on our most recent aircraft, the seats with armrests (which do help) are not fitted, but instead a very short leg, lightweight seat which gives no support to the thighs.
Another of the problems is that the cushions end up compressed so badly after being flown by some of my more "fuller sized" colleagues and not being renewed at appropriate intervals. Management has talked about replacing them for as long as I have been in the company, but (surprisingly), nothing has yet happened.
The most recent contribution to poor seating posture has been the introduction of lifejackets with a built in air supply. The bottles are not light, and even after short flights, pilots are complaining of back ache because of the lack of arm support which helps keep the spine straight.
As usual, its the pilots, who can sit in these seats for up to 7 or 8 hours in a day, get the cheapest option available.