The stuff is very hard to spot from the air. Many glider landings were planned for less than full daytime. The glider’s landing approach angle is shallow enough that if there was any wire it’d be real hard to go over one and safely below the next even if you see it coming. Last of all although the gliders didn’t roll/slide very far across farmland after touching down it was far enough that you’d stand an appreciable risk of hitting one during deceleration even if you missed them all in the air. When the outer part of the wing hits one the whip-around is pretty severe; you can injure people and or break equipment loose that crushes people. As always wounded enemy are better than dead enemy; each wounded man takes somebody else out of the fight to help him, at least for awhile.
Or at least that’s the idea behind asparagus. As noted by wiki, it wasn’t terribly effective. What I can’t say is how much that lack of effectiveness was from folks landing amongst it unscathed, or from mission planners avoiding those areas and landing elsewhere. With perhaps those other places being more heavily fortified with defenders. Passive defenses are an economy of force play. The cheaper and quicker to erect the better.
A common army tactic is called canalization which amounts to "steer the enemy away from (or flat deny them) this terrain and of necessity they’ll end up on that terrain instead. Where we’ve set up a jolly fine reception.