Dan (Vrooomed) said it very well. For me personally, I I have found building a set to be fun, buying one- either factory set form or hand collated by someone else- takes a lot of fun out of it. I've also found it's not really too reliable...almost every single set I've bought, including factory sets, have been missing cards.
A large part of the fun to me is making the memories of chasing the cards. When they all arrive at once, yes, there is some memory of that, but it's not as endearing a memory nor do I end up with the same nostalgia for a set I worked hard to complete.
Even when I buy a box, which is a rarity these days, I don't open it all at once. I still have packs from boxes I bought in the 2012-13 season I have not opened yet. To me, it prolongs the enjoyment.
Another factor...which impacts each collector differently...if you just buy a set, you don't have any chance to pull inserts or parallels. For me, I enjoy parallels immensely, inserts not as much anymore as in the 1990s, but sometimes they are still fun. If I just bought the set I wouldn't get them.
VERY slow trading due to health problems. Not transferrable so safe to trade with, just moving is painful and can't always access the cards.
Cardboard History My COMC
New Collection Website: Cardboard History Gallery (Still under construction)
Tips on how to make your scans look like the card does in hand (No more washed out, fuzzy scans!):