The thread Molakule refers to is excellent. Add molybdenum dialkyl dithiophosphate as well. As you can tell "moly" has many forms. The chemistry behind it all is Way complicated and as with all formulations the whole package needs to be taken into account.
Soluble moly with stabilizers has a coefficient friction of .04 to .08. Chevron-tex have done extensive studies showing that Organo -moly soluble compounds can reduce an engines coefficient of friction by up to %80. Similar studies have shown an energy reduction of %5. The use of the proper "moly" is a friction modifier, enhances zddp as An a/w agent and friction reduction modifier. Moly enhances a lubricants resistance to oxidation and is highly acid resistant. Will Plate up on metals up to 1.2 microns. Withstands up to 500,00 psi.
To answer your qusetion why some do and some don't it could be the expense. It takes 2000lbs of mined molybdenite to produce just 4 to 6 pounds of moly disulfide.From there an extensive refining process for the various types of moly and there specfic uses.