Capitalism, kind of. Practicality, the rest of the way. Mobile parts need to be designed really tightly integrated, because they need to fix exactly into such small spaces, and standardizing them isn’t really feasible without significant pressure on the market (aka, socialism).
The reason desktop PCs can be so standardized is how big they are. Tons of room for customized parts.
You do realize that computers and the internet were invented and developed by the government, right?