The Chinese market is different.
Chinese carriers offer minimal subsidies, so most customers are paying close to full retail for their phones and aren't tightly coupled to a two-year upgrade cycle. Used phones are a popular choice for value-oriented customers.
Repair is the norm rather than the exception, so most customers will replace a cracked screen or a weak battery rather than just disposing of their phone. Labor costs are relatively low and parts and documentation are widely available. Broken phones (especially iPhones) are frequently bought by dealers, repaired and resold.
The big Chinese brands offer relatively good long-term OS support, but that isn't hugely important for Chinese users. WeChat is the app in China (for many users, it's the only app they ever use) and it's extremely well optimised for older devices. The APK is a mere 60MB and it'll run smoothly on devices with a dual-core SoC and 1GB of RAM.
The US market has historically been the polar opposite of China, but it's creeping closer. Chinese manufacturers are making a push for the US market and consumers are starting to wise-up to the benefits of paying cash for a mid-range device rather than taking the "free" upgrade when their contract expires. The used market is flooded with two-year-old flagship handsets that will perform admirably for another few years with nothing more than a new battery.