That's because the light is passed through the scope in narrow beams. 30mm scopes aren't any brighter than 1" scopes. The advantage to going to a 30mm tube is that there is a lot more room for the internal parts to move when making very large adjustments. That's only needed in a scope used on a rifle intended for shooting at extreme ranges. But for rifles shooting normal hunting cartridges at normal hunting ranges (<500yds) those kind of big adjustments are not needed. There really is no good reason to use a 30mm scope on the average hunting rifle. All it does is add size and weight. It's a fad that has caught on from people wanting to emulate the scopes on sniper and 1000 yard rifles. Unfortunately, since 30mm tubes have become so popular, there are fewer good 1" scopes to choose from.

If the shooter wants more light, he should use a scope with a bigger objective diameter.