Because he isn't good?
He's not great with ball at feet (he would be better than de gea) but he wouldn't be the improvement eth wants, his shoot stopping, especially long range is poor, he doesn't have the reach, he's poor 1v1 which we get a lot of, while better in the air than ddg he's small and doesn't command his box like a top level keeper should either.
He's just come off the best season in his career. What's to say that's not an anomaly?
You're the one going against the consensus. You should be explaining why he is good to us. Not the other way around.