I have always supported your opinion Schultz technically is a good blocker, but his upper body simply lacks the mass to effectively compete at the next level. His shoulders are too narrow, which means he doesn't fill space. (Compare him to his TE compatriots in the photo in the linked article below, especially to Smith on his left.) The lack of girth means he's easy to cover, too, because he creates no space when a LB or safety is leaning on him on those routes over the middle. Unlike Smith, who is "open" even when he is not, Schultz looks blanketed to his quarterback. He's easy to reach around and deflect balls. If you watch Smith run, he is not very quick or fast, but he's a legitimate target on virtually every play.Schultz is probably more athletic, but he is not the same target.
There's gotta be a reason Schultz is one of the least productive receiving TEs we've put out there in years...and I think the issue is structural, no matter how excellent he is at the fundamentals.