Is it practical to extract zymase from yeast for ethanol production?
So, do people use such a method, and if not, why not?


No, people do not use such a method.  There are several reasons:


In fact, conversion of glucose to ethanol requires many enzymes, not just one.  The crude extract originally known as zymase was not characterized until later to consist of so many different molecules.  The molecules that comprise zymase include all the enzymes of glycolysis (e.g. glucose-6-phosphate isomerase, triose phosphate isomerase, phosphofructokinase, triose phosphate isomerase, etc.)
The other component of "zymase" as it was understood historically included dialyzable components.  Later these were identified as coenzymes, and are now understood to consist of molecules such as NAD+, ATP, and coenzyme A.
Live yeasts can synthesize all the enzymes and co-enzymes they need to convert glucose to ethanol.  But yeast extracts such as zymase can't. As they convert sugar to ethanol, eventually cofactors are used up, lost to processes such as dilution, oxidative damage, or enzyme promiscuity.  When a zymase extract loses its activity because of these reasons, it can never get them back.  But live yeasts are always synthesizing more coenzymes (and enzymes) in a programmed response to any sensed shortage.
Even zymase would not go to 100% ethanol concentrations.  Enzymes denature and lose their effectiveness at high alcohol concentrations.  The concentration where this happens is probably higher than the concentration that is lethal for yeast, but I don't think it is close to 100%.