I sympathize. I am a minimalist myself.

Some travel of course simply calls for more packing. It's not length of time but how much you are doing. If you are female and are going to a formal business conference, an evening ball, and a big hike, all in one weekend, you will need a lot. You might also want insurance for critical events. Maybe a spare suit if you are meeting world leaders at the White House and don't want to be derailed by a spilled drink?

Once you adjust for activities and contingency planning, some people still overpack. That's just a sort of hoarding anxiety I guess.

There is also a threshold effect. If you have more than carry-on level stuff, some people figure they might as well take a big check-in bag and enjoy having more. If you are driving, filling up the trunk is low marginal cost and stressing out over minimizing luggage seems silly.

Around the world travel, nomadism etc. are skilled travel activities. As you yourself discovered, you have to learn to pack right. Some people are just less skilled. I am sure international flight crews and reporters etc. are even more efficient than backpackers.

Money is a factor as well. If your money is really tight, you have to carry more. With enough money, you need no luggage at all.