The longer it takes, the higher the risk. It probably also takes a lot more capital as they would need to finance more products in advance. I can understand that they want to keep their risk at a minimum. It also keeps their products more attractive for collectors.
If it was taking months, or even weeks, I could see the risk. But selling out within a week at the most, there isn't much risk. Obviously they can't determine ahead of time how long it will take, but through trial and error they can estimate. And if it only takes up to a week (and I mean tops), I don't see the issue. Although maybe there is something I am not seeing which Josh or Doug can point out.
As for the last factor in regards to collectors, can't say I feel that is really a good reason. Don't get me wrong, I like my physical games and instruction manuals, there is a part of me that likes knowing I have something that only a few people have, but I don't think they should artificially lessen copies just so the copies sell out faster and make them more collectable.
That said, I am truly appreciative LRG has done what they have, in giving us physical copies where we wouldn't see them normally, and in the hands of those who appreciate them (mostly). And mostly covers those who buy them to sell. (Fucking scalpers, hate them).