Cratecruncher hit the nail on the head. To get the most work out of the wind size matters. Long high aspect ratio blades do two things that relatively short turbine blades cannot. First you can put a lot more area into the blades compared to the shorter ducted fan style. Second the leverage arm to turn the turbine is much longer so the mechanical advantage is far higher.

Finally there's the fact that even with a really big inlet into a venturi before the focused air hits the turbine there's only so much pressure boost you'll get at the turbine before the venturi packs up and won't let more air into the mouth. This is all related to airspeed and the size of the venturi. But at some point if you can't FLOW the air fast enough the venturi will pack up and stall and there'll be an "ice cream scoop" of stagnant or slow moving air stuck in the mouth of the venturi that will ruin the velocity rise. At that point the air will tend to flow around the sides of the higher pressure "plug" and around the sides of the venturi instead of working harder to cram air into the venturi mouth.

Venturis work best when there's a source of suction in the narrow outlet rather than a resistance that needs to be worked on. Mostly a venturi is a flow smoothening device moreso than a pressure modification device. Yes it DOES do both but mostly it's about avoiding stagnant bubbles choking off the flow at the mouth of a airflow opening.