Same reason non-poor people waste money on luxury goods.
1 - People want to have what other people have. Your neighbor gets a new car, you get a better one. Right? Obviously it's rarely that blunt (we're not all Patrick Bateman, after all.)
2 - People underestimate the total cost of ownership of certain items. I can get an utterly amazing phone (computer! everything machine!) for free. And I mean, $100/month isn't that much, right?
3 - A lot of luxuries really aren't that expensive, all things considered. I mean, remember that phone from point 2? You can buy a fairly capable computer, that can crunch numbers, play video games, play video and audio on large screens, call anyone, receive data from anywhere, run any arbitrary code, be a learning tool, be a work tool, and last all day on a single battery charge... often for $150. Are smartphones a luxury? I'd say for most people, they aren't strictly necessary. But a 1MHz computer used to cost $10,000 in today's money. We spend 100x less for something 100x more useful than our parents may have. And hell, you can get older smartphones for free, because people are upgrading and have absolutely no use for their old device. We're very quick to assume someone is spending $1000+/year on a phone when they might actually be spending nothing, and use it as a phone only for 911 calls (free)!
Obviously there are cases of obviously insanely stupid spending, but someone earning $10k buying $300 shoes on credit is about as wasteful as someone earning $50k getting into debt for a $50k car. Yet nobody complains about the second one.