The problem is mainly one of information and time. This subreddit proves the point that it's hard to find out how well built many things are. Even things with lifetime warranties vary in whether it lasts a lifetime because it was well built, or it lasts a lifetime because the warranty replaces it many times.
I'm lucky in that I now work a job that pays well, and I have lots of spare time to faff around on reddit, amazon, google, and going to stores to compare things and try and find at least a decently built option. This is still a little hit or miss, and I dedicate a significant amount of time to this on "important" purchases.
When I worked two jobs, I had no time. It was great for saving money because I never had time to spend it. I was working 8-5 at my main job, and 5:30-9:30 and weekends at my second job. I also didn't have free time where I could play on the interent at work, and at home I mostly slept. So when would I find "stuff that lasts"?
Even today, there are things I can't spend the time to search for a "good one". Or "Good one's" aren't immediately available.
Example - I wear large sizes. I can go to the mall, or goodwill and search for hours across various stores, and maybe find 1 shirt that fits. Or I can go to haband and order 5 shirts in 15 minutes (their website sucks and is slow). The shirts aren't awesome, and don't last forever - but they always have my size.
I recently needed some sissors. I saw the recommended ones from England on this subreddit, and went and ordered two pairs. This was in December. I don't have those sissors yet. I still need sissors. So I had to get some crappy sissors from AC Moores (or Amazon or Wal-Mart or whatever). They already don't cut well, sticking on cutting my vacuum sealed plastic bags and tearing more than cutting. But they work far better than imaginary sissors that I have on order from somewhere in England.
As to people can't afford to buy stuff that doesn't last - it depends. Some people can't afford to buy stuff that lasts - the price is just too high for them to pay. You can't magically find anything you want, at a quality and condition you want at goodwill any time you need it.
Finally, you are missing what seems like a big point to me. Not only do companies design for a lifespan, consumers buy for a lifespan - sometimes fully thought out.
For example - if you're going to make a bookcase - once as a hobby project - you may need a hammer. But you're only going to use it for like a month. It makes sense to buy a cheap hammer because you're going to be done with it quickly. Maybe you don't have room to store it, so you'll be throwing it out afterwards. No reason to BIFL cause you're BIFa month... Or even if you're going to keep the hammer - I've seen plenty of people have cheapo hammers last 50 years because they use them once a year. Could they get a much better hammer? Yes. Would it do anything other than waste money? No.