Because PC is an American thing but phones are not. Obsession for standardization, modularity, and cross-compatibility are rather unique American cultural traits that aren't nearly as strongly manifesting elsewhere. "Fits right in" is quintessentially American thing.

The entire unitized jet engines on Boeing aircraft drops right off and swaps right into another host, sometimes even to different types of aircraft. PCI soundcards come off a i386 PC and go straight into PPC Macs. AR15 pressure bearing parts don't merely interchange between examples from different time and place but its grip and stock mounting patterns are becoming a industry standard of its own. Early Tesla battery packs come apart into bunch of 18650s and could reassemble into new packs(though it's a big no-no due to RUD risks). Meanwhile, Prius power units or front seats are for Prius only; it won't go into dozen different Toyota models, at least without substantial parts changes, modifications, and reconfiguration. Bugatti Veyron uses its own custom tires that aren't even forward or backward compatible with their own successor.

Same for phones: .apk runs everywhere, Linux do not, cameras don't interchange, internal connectors don't fit together, LCDs specific to anything are default unobtainium. microSD cards works on everything, but the moment you look away, Huawei invents a new incompatible format for absolutely no reason. Apple "reinvents everything" every time but internal organizations of components are stable at macroscopic levels for few generations unlike most other manufacturers.

It's openness of PC that is unique and precious, not closed nature of everything else being odd and inconvenient.