One theory I've heard for this, and sorry I don't remember the source, is that Napoleon suspected if he truly did take down the Russian royalty then he expected the rest of European royalty to unite and attack attack him.
Also what surprises me, after years of several revolutions and chaos in France, how could Napoleon gather such a large army.
[1] https://en.wikipedia.org/wiki/War_of_the_First_Coalition
Conscription + France was basically the China of Europe at that point: it was almost as populated as the rest of Europe combined.
France then had a very, very, early demographic transition which dramatically limited its population. Had France followed the demographic path of England or Germany, France would have around 250M inhabitants today.
That doesn't seem to be correct. From Wikipedia: "During the Middle Ages, more than one-quarter of Europe's total population was French;[8] by the seventeenth century, this had decreased slightly to one-fifth. By the beginning of the nineteenth century, other European countries, such as Germany and Russia, had caught up with France and overtaken it in number of people." https://en.wikipedia.org/wiki/Demographics_of_France