It's not (entirely) arbitrary. Speech is variable, our vocal tract shape changes all the time, but not arbitrarily fast. From a source-filter theory perspective

It's not (entirely) arbitrary. Speech is variable, our vocal tract shape changes all the time, but not arbitrarily fast. From a source-filter theory perspective it does make sense to consider speech production over short time frames as linear time-invariant systems. What this has to do with 44.1 kHz is beyond me, however..