Because the creators have probably never seen an accurate cowboy
Need more info in order for someone to come along and answer your question.
This shows when Napoleon visits on March 11, 1799 for his sick soldiers which were located at the Plague that Strikened in Jaffa, It was an attempt to put down shady rumors after Napoleon ordered that 50 incurable dying plague victims in Jaffa to be poisoned during his retreat from his Syrian expedition.
This is what basically the painting was all about.
