Well, I know why. He, and many other hobbyists become desperate when fish get sick, and they have no QT.
In this hobby, you must bite the bullet and set up a QT. Believe me it isn't as painful as it seems, and once you do it, you can finally breath a long sigh of relief. I have yet to see hobbyist claim they regret it. Imagine being able to RELAX and ENJOY your new fish when you bring it home, rather than sweat bullets for 3 weeks wondering if you are going to have a disease wipe out. We come into this hobby because of the pleasure it gives us, so set up a QT and enhance the "fun" part of the hobby. QT's can actually be interesting, dare I say.