Glass back is a premium design feature because it breaks easily, and customers who buy premium phones are expected to be rich enough to just buy another phone.
It's a profit making feature.
Much like luxury cars, nobody actually expects BMW or Mercedes to last more than 3 years. People who buy them are expected to trade up for the latest model every few years.
Same with luxury fashion. Absolutely some of the cheapest and fragile clothing I've ever seen come from big fashion brands. And nobody cares, because by the time they break, they're out of style and the buyer will be updating their wardrobe anyways.
If there's one thing I've learned, expensive luxury items = cheap and breakable. Midrange products are where the customers tend to be concerned with longevity and value.