But they are a symbol of the wrong social class. A musket is something that a peasant or a burgher can use to kill a noble. All the main characters in the three musketeers are nobility, and their social class has suffered greatly from the "democratization" of war. They, like almost everyone like them historically, much prefer the old ways from when they were more pre-eminent, and look down their noses at firearms. They spend very little time at war, and a lot more time duelling and participating in schemes.
The high-tech of the early 17th century wasn't even matchlocks anymore, it was flintlocks. Those took another ~50 or so years to become general issue, but at the time of the novels upper class people who can afford modern weapons wouldn't have been fumbling with matches anymore.