Because some people think LV bags actually get better with age, like fine wine. If you buy them brand new, they will just end up with that honey color sooner or later, so if it's easier financially, why not save a few hundred bucks.
I've never owned a LV--how long does it take for patina generally? I have a coach wallet that has a vachetta interior--I've had it for almost 2 years and still no "real" patina--but it's also on the inside and not the outside....
I've noticed that if you live in a cold climate and you're not out in the sun much in the winter, the bags take a really long time for the trim to darken. I have three new LV's with the light leather and they are not changing very quickly because I live in NH - trust me on this - the snow reflection does nothing to darken the bags!!
One of the 3 bags was changing nicely (my Hudson) but I had to have it replaced, so now I have a brand new one with the super light leather, so I have to start from scratch. I'm going somewhere sunny next week, but only bringing one bag, so at least I get to decide which one I want to bring and darken up.
I'm not really complaining because I could have much worse problems, but I know what you mean about the patina. Once there's a patina, there's less worrying about water stains too.