He's the safety valve receiver by default, because he's the RB on the field. That by itself doesn't mean he's good at it.
Let me turn around your question: "Why would he do so little with all those targets if he was an above average pass catcher?" And no, Dak isn't dumb. When you need to dump it off to your safety valve, that's what you do. It's better than taking a sack or throwing it away, even if your safety valve doesn't do much with it.
It's the exact opposite of what you claim. We look at Zeke's performance as a receiver and say, meh, he's not been very good compared to all the other RBs. We look at Witten's performance and say, wow, he was very good compared to all the other TEs.
It's possible there's a stud receiver hiding inside Zeke that's been stifled by the playcalling. But until I see it actually show up in performance, I don't believe it. Which is okay. He doesn't need to be a very good receiver to be valuable. I just don't get why so many people are so invested in him being one despite all evidence.
Dak part was in satire.
It really depends. Advanced stats show one thing, game tape shows another. Safety valve isn't usually a big play receiver. Like you posted, it's the last option, usually close to the LOS. They are schemed to be wide open. Usually out in the flat or in the middle of the field, where depending on the coverage run, there's a defender there in close proximity.
Witten like Zeke(rushing the ball) is a volume player. He racked up stats by getting force fed the ball at times. Never had great TD totals or YPC. And from 2010 on, he slowed down quite a bit.
But my point was recently Witten was not an explosive TE after the catch. He knew where to be, but usually turned and fell or rarely got RAC.
I don't care to over inflate Zeke's value. I'm of the opinion that you can get a good back in day 2.
But, as someone who has received a lot of targets recently, he has been good at actually catching said targets. Why Garrett hasn't schemed more more in the passing game, who knows. Gone were the screens from 2016. Maybe they knew he was running the ball a lot and didn't want to have him as the primary receiver on passing plays moving forward. If Zeke led the league every year in drops, over various sources, I would agree he was a below average pass catcher. But he has shown here and there he can be used more as a safety valve. And as the safety valve, he has done a good job at catching the ball.