halacha theory - logic behind kol kavua k'mechtza al mechtze

there is a principle in halacha regarding doubts that if something is separated from a kavua (fixed place) it is treated as 50/50% chance even if the permitted is the majority.

but if it is found in the shuk then it goes by the majority.

so for example, if one finds a piece of meat outside and most stores are kosher then the meat is considered kosher.

but if one takes it from a store and it is unknown which store then it is 50/50 doubt even if the majority is kosher.

does anyone know whether there is a rational argument for this or is it just straight halacha without logic?