They lack vertical integration and are willing to fleece their clients thanks to their position.
Apple's vertical integration means that they can plan chip design in cooperation with final product teams, which means many cost-affecting decisions are made with budget view that includes the whole device and its sales, not predicted markets among chip makers whose total budgets for device designs aren't known.
Apple can easily decide that they are willing to spend a bit extra on, for example, large caches (which have HUGE impact on performance) or very wide superscalar cores, because they are not going to get clients complaining that the chip is now too expensive - and the specific guidance on what design parameters should change is informed from start by the target product.
Meanwhile the closest relation to Apple's chips, Samsung Exynos, for various reasons had its team blocked for cooperation with Samsung Mobile part that designed phones - they effectively had to make most of Exynos SoC in vacuum targeting general markets and hope that selection process doesn't decide Qualcomm's patent and familiarity advantage beats them out resulting in bad financial results which in turn gets management to look badly on them with obvious consequences (I remember how just the fact that they used PowerVR like iPhone at the time led to complaints because games were tested on adreno only).
Neither Qualcomm or Nvidia are providing deep integration at that level (from my understanding, even supposedly "custom" stuff like Qualcomm's S1 and S2 are nowhere near as integrated in design phase as any Apple chip - and Qualcomm seems willing to break Microsoft design for the entire platform with what they delivered). And it was not uncommon to find out way too late that the chip that was selected earlier doesn't really deliver (I recall such complaints specifically about first Snapdragon being massively slower than expected by Google Nexus team, and similar case happening later with Tegra in Xoom. There might have been suggestions by people I talked with that HTC for, possibly informally, banned from consideration by Google Nexus team for a time due to that - Xoom supposedly also made for a bitter pill with nvidia my understanding is that Tegra GPU architecture had some fundamental mismatches with Android)
