Lecithin, the emulsifying agent in egg yolks, is not the only emulsifier that can be used with butterfat. You can substitute the milk protein casein, for instance, if emulsion is your only goal. (In fact, the small quantity of casein in regular butter does, to a slight degree, help emulsify your homemade hollandaise sauce).
Hollandaise sauce, however, needs egg yolks because they perform other functions. They contribute flavor, color, and nutrients and provide some water; if they did not, you would have to increase the amount of tap water called for in the recipe. Yolks supply a little fat, which supplements the butterfat.
Finally, some of the ingredients in the yolk work as a team to enhance the stability of the sauce by increasing the sauce's viscosity.
** Asian Recipes