Because once you've cooked them theres hardly anything left, all the water has gone so *by weight* all that is left is almost pure carbohydrate ie calories. Like the crisp example: a whole potato might fill you up, but a potato's worth of crisps - with all the calorie-free water etc sucked out of it - would not. If you start snacking on these they're not going to fill you up, you're still going to be hungry and end up rooting around for more food.
If you can control yourself to stick to say 30 chickpeas max AND eat it alongside plenty of superfree food etc as a meal, fine, but most people won't.
I'm not saying I agree with any of this but it seems that's the way SW teach these days.