After witnessing the affects of predators, most people do bring them in. If you bring them in, you can watch the cycle more closely. But, inside there is a whole range of problems to. Don't overcrowd, keep leaves dry and humidity down.
Sometimes it is just the time it takes to raise them. Many people don't have enough time and so they take the time to take care of their gardens and then let nature take it's course.
I think I saw somewhere that a Monarch can lay up to 600 or more eggs, but only 2 will survive to adulthood. I joke with the visits I give butterfly walks to that I love butterflies, but I love birds too. Birds eat loads of caterpillars. Not eating Monarchs is a learned response so they have to eat at least one to know not to eat them.
Herndon, VA (USA)
Take care of the small things....