I can't say definitively why, cheshire, but different words have different "implied connectors".I'm curious to know why you can't omit "to be" in "I pretend to be a genius." but you can in the following sentence.
"I consider him an artist." There is no "I consider him SV." form, there's no possiblity of coming a verb next to "artist."
"*I pretend an artist." There is "I pretend (that) SV." So, it leaves ambiguity.