I don't know what you mean here, I can plug a speaker into my mic slot and use that to record, just as plugging a mic into the speaker slot gives a (crappy) speaker.
A DAC and amplifier circuit is electrically incapable of processing input (on its own.)
Physically unplugging and moving a speaker to a mic input works, sure, but very few devices can do this switching electronically.
Because on your computer the engineers purposely put a switch that can direct the signal to either the input hardware or the output hardware.
It's not a mic slot, it's a general analog I/O port with a 3.5mm form factor.
> purposely built input stage
You moved your device to the purposely built input stage.
Not an expert, but your remark doesn’t compute with the parent comment