Because the novel by Dumas is based on Mémoires de Monsieur d’Artagnan, capitaine lieutenant de la première compagnie des Mousquetaires du Roi (Memoirs of Mister d’Artagnan, Lieutenant Captain of the first company of the King’s Musketeers) by Gatien de Courtilz de Sandras which was then based on the Musketeers of the Guard who were created when one of the King Louis’s created a company sized element of light cavalry equipped with muskets. And besides, no one reads about musket battles, so boooooooooooooooooorrrring.