The problem is that desktop computing is based on mouse interaction, and the precision unit is the pixel. On touch-based devices, it is the human finger. The actual physical dimension of the buttons and other UI elements, including spacing, start to matter a lot more. When you allow for a huge range of resolutions all at different physical dimensions, it's nearly impossible to craft a good UI for all of them. It's going to be a lot of work (too much for most devs) to verify that their flexible layout is going to work from 320x240 to 960x540 and remain usable and readable. Because it's so hard to craft the perfect experience for all, most just don't bother and just settle for good enough. In the end, there are huge benefits to the way Apple is doing things from a UX perspective.
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The problem is that desktop computing is based on mouse interaction, and the precision unit is the pixel. On touch-based devices, it is the human finger. The actual physical dimension of the buttons and other UI elements, including spacing, start to matter a lot more. When you allow for a huge range of resolutions all at different physical dimensions, it's nearly impossible to craft a good UI for all of them. It's going to be a lot of work (too much for most devs) to verify that their flexible layout is going to work from 320x240 to 960x540 and remain usable and readable. Because it's so hard to craft the perfect experience for all, most just don't bother and just settle for good enough. In the end, there are huge benefits to the way Apple is doing things from a UX perspective.