Because it's relevant!Andy Carroll went for 35M, 2 years later Van Persie went for 24M. Eden Hazard in last year of contract went for 100M+, Sane 50. Hazard also is not twice the player. Hundreds more such example we can give.
Transfer fee and player ability, especially when comparing different transfers have absolutely no relation. I have no idea why we get comments like these. Players transfer value depends on how hot player is in transfer market, his contract length, negotiating power selling club holds, buying club's desperation, buying club's financial status and some more factors. Player's ability is just 1 factor.
A players worth and the valuation a club puts on the are 2 very different things. In the current market his valuation is way off.