Usually that's because the engineer is not a very good engineer, at least outside their specialty. If they had studied engineering more, they would have probably learned that other engineers are already doing the thing they proposed.

"Using methodology from a field known for its focus on methodology in another field" was still a novel idea in von Neumann's time. But because von Neumann and others did it, and because millions of people entered science after them, most combinations like that have already been tried. Breakthroughs from combining ideas from unrelated fields require something little less obvious these days.