She wants to do it her way, which will only mean he'll end up in the phantom zone, or take Clark with him. Or possibly have to be killed by Clark or they both kill each other. Her stupid choice will cost more lives than they will save. By putting it off, he'll only get stronger.
Perhaps that was her intention. Give him enough time to get strong enough where there is no other possibility. Maybe she'll sacrifice herself for this mistake. So that Clark can become superman. And Chloe is gone...