Since the largest desert cities are also the newest big US cities it makes sense that the skylines are still immature, especially compared to old eastern cities like Boston, Philly, etc.. Arizona has only been a state for a bit over 100 years, although the area has been inhabited for 1000's of years.
Phoenix is beginning to add quite a bit of density now and is growing into it's own. The airport location limits downtown Phoenix height but the sheer quantity of buildings are growing substantially. Here's a recent photo I took.
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