Because there's no value in overengineering things that are easy to replace and where the consequences of failure are trivial. Further, most people only need the features of their phones to be "OK" rather than "GREAT" and would rather carry one device rather than 10.
For some things - such as clothing or furniture, or items where there have literally been no earth shattering developments in the last 100 years (like, I dunno, silverware), it's okay to overengineer because doing so is actually efficient. I have a coat and a pair of boots that have lasted me 20+ years, some silverware that's maybe 200 years old, and the average of most of the "important" furniture in my home is over 75 years.
But my phone? I'm not a professional photographer. I'm not even an amateur photographer. I just want pictures I took of people and things and events I found worth photographing that are "good enough." I'm not doing professional video editing, so I just want a video cam that's good enough I can take footage of my dogs doing goofy stuff that I can send to my family. If I'm in a place where I'm watching movies or TV on my phone, it means I'm traveling and therefore unlikely to give much of a shit if the screen doesn't have perfect color fidelity or whatever because, well, there's a bunch of shit going on around me anyway. Ditto for music - why would I aim for some kind of audiophile's wet dream when likely the only time I'll be using my phone for music is when I'm out and about in situations where music quality isn't terribly relevant? Etc. and so on.
It's not that we don't value quality - I think we DO value quality very, very much - it's just that we can recognize that it's kind of stupid to waste time and money and effort on overengineering things that will be hopelessly outclassed in a few scant years.
Buy quality where it matters, buy cheap and replaceable where it doesn't.