Because we paid £52m plus bonuses to Real Madrid on the basis of a season where he was one of the best players in the world and crucial for them. PSG will be buying him after terrible season in which he lacked motivation and played very badly in 80 per cent of games. Also, he's 27 now and will be turning 28 in 6 months time and a player who's heavily reliant on pace will always take a hit when he passes 30 - there's depreciation here and I'd actually say that it's progressing even quicker for a footballer between 27 and 30 than for a car. 26-27 is still reasonably young, 28-29 is pushing it and 30 is an age when you no longer pay record fee unless it's Ronaldo/Zidane/Messi.
Baffles me how people think that his value should stay the same when he's been crap for almost a year. Actually I genuinely feel that PSG are the ones being mugged here if they're paying £46m for Di Maria. PSG are bailing us out really, £6m loss on a player we overpaid for who has been ridiculously bad towards the end of last season.