Historic reasons. AMD developed AMD64 based on x86, while Intel worked on a new 64-bit platform, which never really took off. They later released CPUs with AMD64 support, but never called it that for obvious reasons. The first name stuck, which is why most projects still use it.
(This is going a bit off-topic, btw.)
well yeah, I still suggest to just use 32-bit and 64-bit as extensions especially since this thing is made to be easy and the AMD64 is just confusing, but anyway as I said before there are things that are more important, like for example building PHP into caddy directly.
@My1 it’s off topic but just write this echo %PROCESSOR_ARCHITECTURE% in a cmd console to see what your Windows says about your Intel processor architecture 
I’m going to close this thread so that people aren’t tempted to use it for Caddy questions rather than starting new threads!