Why are some electronics seemingly unbreakable (phones) and others very fragile?
When looking at an iPhone logic board, it is clear that it is designed to withstand a lot of abuse. The critical and usually larger IC's, such as the SoC, PMIC, NAND are usually encased in a strong underfill while some of the passives, especially those near FPC connectors, are also covered in a softer epoxy. The vast majority of the passive devices are 0201 or 01005 and most of the IC's are micro-BGA. All of the connectors are secured with brackets so that they don't shift or get dislodged, possibly damaging the connector or surrounding components due to an impact.

So clearly there is a design side to it but I would also consider some basic physics as well. Modern smartphones are pretty light and often times inside a protective case. This makes the force of an impact much lower than other device types. For example, if an iPad or similar tablet fell from the same height, it would be more heavily damaged.