Why do people carry "Big, Heavy" packs?
First, most of the gear I see here I had never even heard of before. You'll never see the stuff discussed here in a Sport Chalet or REI (the two retailers near me). UL gear suffers from a problem of obscurity to the general public.
Second, BUDGET, BUDGET, BUDGET. Before I was making decent money, I'd do garage sales, cast offs from friends, sales, whatever. Beggars can't be choosers. The most common stuff is, you guessed it, heavy.
Third, it's a BIG leap of faith to start buying gear over the internet that without trying it on/out first. I've wound up with stuff I didn't like. Yeah, I can Gear Swap it -- at a loss. I see a lot of people here who are super experimenters, constantly striving to lower their skin out weight. Most of the people I know won't take/don't have the time to do that.
Fourth, it's just plain hard work to lighten up. I bought a lighter pack, a lighter pad, a lighter bag, and a lighter shelter. I struggled to get my pack weight down to 38lbs (total weight incl. food, water, and fuel), and I made some compromises that left me a little nervous (carrying an Ur Sack instead of a bear canister in Sequoia Nat'l Park for example).
It's a huge, nay, HUGE change of mindset for someone like me who started hiking and backpacking in the 60's. It ain't easy.
Having said that, my 50lbs days are THANK GOD over and I can't see myself needing a pack as large as the one Ryan posted even for a winter trip.