>>18148534 (OP)
Because they had much bigger populations.
France is often noted for havinga particularly large population in that era (which is true), but it didn't come anywhere near close from the total population of its enemies combined.
In 1800 France proper had 26 millions and when counting its annexed lands it reached 40 millions
Meanwhile, Russia had 28 millions, the HRE had 40 millions, Prussia, Britain and Spain all had about 10 millions each, Naples had 7 millions...
France never stood a chance on the long run.