Having to rebuild rarely has anything to do with the instruction set - the majority of processors use ARMv7, and building for x86 or ARMv8 instruction set processors is done regularly.
Otherwise, you're confusing the pc industry with the mobile industry.
Can you actually buy an i386 architecture chip any more? That's a full 30 years old.
If you had a machine with one, what OS and what apps would you expect to run with what performance? As for standardization, the AMD and Cyrix chips of the time, while instruction-set and even pin compatible with the i386, didn't run apps the same way under the same OS.
Anyway - it's true that there are processor variations and that leads to different build options for the kernel as well as some system libraries - and so does the other hardware variations that each OEM adopts for competitive purposes.
The Windows PC market was driven by price right from the beginning and all along its evolution - copy hardware for cheaper without violating patents and just run Windows. And oh, by the way, make it the user's problem to get drivers installed right - there's a grand idea that everyone just loves (said no one ever). And cater to Windows whose deployment philosophy has always been - screw the user, we'll take as much of the hard disk as we please.
Compare that to a diskless, real-time operating system where there can be no BSOD or kernel panics tolerated and where the only user add-on to change configuration might be an SD card. And no target for a single hardware platform driven by lowest price.
And you nailed it with SoC - system on a chip. A huge slice of what was once a collection of chips is reduced to the same collection of different cores and logic functions - but now it's on one chip.
And then comes the fact that not all compilers are created equally.
I think that leads to a full build if you want to get it right.
