Waste of a lane? How is it any more waste of a lane than anything else? You are also in a prime position to massively fuck up your team's XP and gold with failed ganks while leaving your mid opponent really fed. Of course you will "generate more gold and xp" if you manage to get tons of kills - that goes for everyone. By the same logic everyone on the map should be ganking nonstop. But they aren't, for the same reason that a mid shouldn't. Indeed, when competitive games have a dedicated roamer, he is very, very rarely a mid.
"Just go and kill everyone" sounds like really good advice, but sadly it's far from that simple. A mid will also get detected easier by wards if he goes ganking... and that is a lot of gold and XP wasted if you walk down or up to a lane, stand in the jungle waiting for an opportunity.. but the enemy plays too safe, either because they know you're there or for some other reason. Then you have to run back. Meanwhile, your opponent is up ~600 gold and 750 XP.... and you're down the same amount. Then if you gank again and it doesn't work out, suddenly it's double that. And so on.