[pwg]

> Why do smartphones and SoCs require a custom build per model unlike PCs?

Because there is no standard to which the SoC's adhere in the smartphone market.

PC's all adhere to the "IBM PC" standard (well, as extended along the way) so they are all similar hardware.

There is no such similarity in the ARM SoC smartphone market. Every variant of the SoC is different, such that different support is needed.

Now, as to why, that is a different question, and one to which any answer is just a guess. Most likely generic guess is because there was no need to be compatible with a standard from the beginning, and now they are all 'custom' in their own divergent ways.