Because, plants respond to stress by building tissue. If the logic followed, then how would body building work? It stresses the body, actually destroys a little bit of muscle tissue, yet the muscle, all one muscle, gets BIGGER. You don't grow more muscles (number of muscles), you grow BIGGER muscles.
When you top you force apical branching growth. In other words, where you had ONE growing tip you now have two. Now, how that translates into less yield, I don't really know. But, I do have an idea of how auxin works, how light affects yield, and how topping at the right time definitely produces that apical branching I'm after.
I should take some pix just to show you guys what I'm talking about. The difference is quite notable. Because I'm forcing this side (apical) branching, the plant is growing outward as well as upward. It's not like a person, though, in that if you keep feeding a person they can only grow so much so quickly, a plant has more flexibility than that (obviously, within its genetics). However, I have yet to run across any other plant that does not benefit from pruning, and I have yet to run across any other plant that does not respond to pruning by GROWING, often remarkably so. It's not as though you're cutting off its finger. You're doing something that it's evolved to "expect" in nature.
Do we need the pix to see what I'm talking about?
I am growing three strains; Papaya; California Indica x Big Bud, and Conquistador. My son is growing AK47, Master Kush, and... now I can't remember what else. One or two or maybe even three others.
I could take the topped bits and clone them, but I am not currently set up to do that, and this is not a plant that takes to cloning very easily like, for instance, geraniums, because I've tried it the way I root other plants and it hasn't worked.