Answer:
The main difference between a salary and an hourly wage is that when someone is being paid monthly it means that their salary is fixed, meaning that the amount of money they get at the end of the month is not prone to changes. On the other hand, we have a wage earner who is paid by the hour of a specific work they do. It means that if they work 40 hours per week, they will get a certain amount of money but if they work an hour or two longer, they have the right to recieve that extra pay.
Because we Americans don't want the government to come along and make some random law about taking our self defense away from us.