Because stats don't tell the full picture do they. Stats tell part of it but the best way of judging a player is by their contribution and what each fan sees from a particular player.
What I see from him is an excellent shot stopper, who is one of the best in the league in that regard, great at saving penalties and is fairly young. The negatives to him are that he is too over confident with his kicking and is prone to mistakes. But find me a goalkeeper around that isn't prone to mistakes. And those keepers who do make them less often, wouldn't even entertain the idea of playing for a post 1987 Everton FC.
The stats about how many he conceded include his time for Sunderland, a team that was the laughing stock of the league and went on to suffer a double relegation. I wonder how David de Gea's stats would look if he played for that side?