Why is it difficult to determine which countries belong to Europe and which belong to Southwest Asia

Countries like Turkey and Russia span both Europe and Asia, and countries like: Ukraine, Belarus, Latvia, Lithuania, Estonia, Moldova, Kyrgyzstan, Uzbekistan, Tajikistan, Turkmenistan, Georgia, Azerbaijan, Armenia, and Kazakhstan comprise the former Union of Soviet Socialist Republics (U.S.S.R./Soviet Union; Russia effective December 26, 1991), which as a whole spanned the continents of Europe (to the west; eastern Europe) and Asia (to the east; western Asia).  

The following independent republics from those listed above that are located in Europe include: Ukraine, Belarus, Latvia, Lithuania, Estonia, and Moldova.

The following independent republics from those listed above that are located in Asia include: Kazakhstan, Turkmenistan, Kyrgyzstan, Azerbaijan, Tajikistan, Uzbekistan, Armenia, and Georgia.  

In general: bodies of water (including oceans, seas, and bays) serving as separations (not borders) between two or more countries, in turn separate one continent from another on a world map.