Well, headphones also come at reduced RRP too. That point can be made about anything. Someone with 30" monitor who only play casual games (and other productivity work) where a 4830 will be fine may still wonder why anyone would spend several hundred pounds just to play FPS.
And while it is true that headphones do go into the thousands, the median for upper-end headphones is much lower (though yes, it goes even higher than £500). The Sennheiser HD650 (and HD600 before that), flagship product until the recent HD800, could be bought on Ebay for under £180 at one point several years ago. It's still an excellent product today in spite of the fact that Sennheiser (and other competitor) has released a higher end product. The same can't be said about a £180 graphics card bought 5 years ago.
I have to say though, I spend more time listening to music than I play games (and of course, I need sound/music when I play games and watch movies).
[I should also note that just because they are known to be 'rock cans' doesn't mean they are only good for that - same thing with high end graphics card which are somewhat or at least often associated with FPS].
In the end, we are trying to put a price on 'enjoyment', which is different for everyone. Certainly past a certain point, I strongly recommend people to try before they buy so that they know whether it is money well spent in their opinion