De Gea's distribution is poor. He's in a system where he's not asked to distribute much, but still leaves something to be desired. He ends up hoofing it whenever the CB's get pressed, he rarely dares a chipped pass to the fullbacks or defensive midfielder like most keepers these days would do. He's the equivalent of the midfielder who passes sideways or backwards 5-10 yards constantly but has 95% pass completion.
David Preece is on the Football Weekly podcast every now and then, and while I don't really think he's all that as an analyst, he summed up De Gea really well in saying he's a goal line keeper. He's the best in the world on his line, but he's still not the best at coming for crosses (that's cost United a few goals this season) he's shite at any kind of mopping up/sweeping, he's not a great distributor.
This is why Neuer is better, and why a keeper like Ederson will be seen as better once his shot-stopping gets enough attention, which always takes a few seasons by virtue of him not facing many shots.